OP posted a picture of Celestia on the casting couch. 
The following is what I wrote based on the idea of Celestia being a well-meaning volunteer porn star.

---

>Celestia awkwardly bounces up and down on the sofa.
>"Oh!" she cries, "oh yes, baby, that is wonderful! Do that! Keep going! Well done!"
"Uh, cut," the director says after a while.
>The action grinds (pun intended) to a halt.
>Celestia gives him a weak smile.
>"Was that not good? Was I not encouraging enough?"
"No, it's the wrong sort of encouragement. Can't you, like, talk dirty?"
>"What do you mean?" her expression is completely innocent. She's not kidding.
"You know. Swear a bit? 'Oh yeah, fuck me harder', like that?"
>Celestia looks at the director like a mother about to scold her child.
>"There's no reason to use foul language like that, Cloud Surfer, especially during such an intimate, loving act."
"Cel... Princess, you're being rutted on a sofa by a complete stranger. For money."
>"And that's an excuse to say foul things to him? I should hope not!"
>She looks down at the poor bastard beneath her, his cock still lodged firmly in her sex.
>"I refuse to swear at you. I don't think a hard-working stallion like you needs to hear such awful things. For what it's worth, I think you're doing -wonderfully-, my subject."
>The 'subject', a young stallion called Ruck who'd only been working in the industry for about seven months, doesn't know how to respond.
>"R... really?" he says.
>Celestia's angelic smile could light up the night.
>"Of course! You're doing an excellent job. I'm very proud of you, and you should be proud of yourself."
>Ruck stares at her. Then starts whimpering with tears in his eyes.
>The director rubs his eyes with a foreleg and lets out a weary sigh.
>On the sofa, Celestia hugs Ruck, patting him on the back and shushing him as he bawls his eyes out and confesses that he never really got to know his mother before she died when he was a colt.
"I think this is the wrong profession for you, Princess."
>"Nonsense," she doesn't look away from her charge, "If this industry is full of broken souls like Ruck here then I'm exactly where I need to be."
"Why couldn't you just be an impressionable college girl..." the director grumbles.

---

"Alright, so you're a lonely, physically neglected royal who's desperate to get fucked, and--"
>"Language."
"You... want someone to love you, then."
>"Better."
"And one day, you give in to your baser urges and invite one of your guards into your chambers. Then, you f-- love, the life out of him."
>Celestia jerks her head back.
"No, not literally, you're not killing anyone."
>"Thank goodness, I was about to get angry with you!"
>Cloud Surfer had never met a Princess before Celestia had wandered into his studio unannounced and wondering "What pornography was, and could he please explain it to her."
>Having only heard stories of their powers, and seen images of them in the paper or on television, he hadn't had enough exposure to appreciate their presence.
>Her physical size had been daunting at first, but he'd gotten used to it quickly.
>Even so. Celestia casually saying she was almost angry was enough to send a shiver through him.
"So, uh, that's the scenario."
>"I think it's a very charming scenario. It's been a while since one of my guards and I had a heart-to-heart."
>Cloud Surfer's ear flicks, and even a director as experienced and jaded as he could appreciate the subtext of that statement. To his surprise, his cock twitches in its sheath.
"Come... again?"
>"My guards and I. We've always had a close relationship. I look forward to seeing how this performance plays out."
>Cloud Surfer dares to dream that this could be the big break.
>A licentious grin forms.

*

>Celestia is reclined on the set's tacky heart-shaped bed with her eyes closed.
>A young mare applies make-up, or at least she's trying to.
>Her hoof is trembling so much that every time she gets close to applying a bit of rouge she pulls back for fear of making a mistake.
>"S-sorry," she mutters for the fourth time.
>"That's alright, Morning Glory, you don't need to apply any make-up if you don't want to."
>"I-it's my job, Cel-- Princ-- Your Highness."
>"I'm sure Cloud Surfer would understand if you're having trouble. I do have an unusual face compared to most ponies, so I imagine your regular techniques wouldn't fit my mask, so to speak."
>"Are you... sure?"
>"If he asks, I'll tell him you applied my make-up so well that you can't even tell I'm wearing it."
>She gives Morning Glory a secretive wink, and it draws a smile from the little mare.
>Morning manages a shaky nod.
>"A-alright, then I guess you're done, Prince... um."
>"Call me Celestia. It's alright, we're co-workers!"
>"Cel... estia? Can I ask you something?"
>"Of course."
>"Are you actually doing this? Like, as a job?"
>"I am still a Princess, Morning Glory, this is just something I'm exploring in my free time."
>"But... why?"
>"I've always made a point to take an interest in burgeoning industries. Equestria has never had anything like pornography before. I must say, I approve whole-heartedly."
>Morning blinks. She isn't sure how to answer.
>"Really?" she says at last. "-This-?"
>"I think it's lovely to have actors perform passionate stories like this. Two lost souls, looking for intimacy, yearning for fleeting comfort. I'm hardly lost myself, though I have had my darker moments, but it's still nice to participate. Did you know, that when train-line technology was getting off the ground, I spent a month as a volunteer conductor? I think it's proper for a Princess to get involved in the affairs of her subjects. It prevents her from becoming aloof."
>She lowers her head next to Morning's with a distant look in her eye.
>"I've made the mistake of being aloof before," she says in a quiet, confessional tone.
>"So... what--"
>A toilet flushes, a door bursts open, and Cloud Surfer storms out.
"Glory! Is make-up done yet? What's taking so long?"
>"Uhh! S-sorry, boss! It's all done! See? You can't even tell!"

*

>Celestia watches the door like an eager dog waiting for her master to come home.
>Ordinarily such a thought might make Cloud Surfer hard, but he has a sinking feeling he knows how this is going to play out.
"Hang in there, king," he mutters to himself, "think of the sales."
>He sits up and barks at no one and everyone.
"Alright, lights ready, cameras on. Action!"
>The door to the set opens, and a reedy-looking stallion wearing an ill-fitting suit of armour walks in as casually as he can.
>Even from here, Cloud Surfer can see his legs trembling. Hopefully the cameras don't pick up on it.
>"You wanted to see me, Princess?" says the actor in a deep voice that's obviously put on.
>Celestia sets down a scroll and takes off her rimless glasses.
>"Ahh, Brass Tacks, thank you for coming on such short notice."
>"Don't worry, Princess, I'll always -come- for you."
>Cloud Surfer winces and wrinkles his nose.
>He shouldn't complain. He doesn't hire porn stars for their subtlety.
>"Your armour isn't up to standard, solider. You should take it off and let me look at it," Celestia says in a crisp, clear voice.
>Surfer's brow furrows and he glances at the script, flipping through a few pages.
>That line wasn't in there. Did she make it up?
>He looks up at the actors. The stallion is pulling off his armour with a confused look, like he wasn't expecting things to escalate so quickly.
>Celestia helps him out of an awkward strap with a delicate tug of magic.
>She turns a backplate here and there, surveying it with a critical eye.
>"I shall have to have a word with the royal armourer. This set is flimsy, it's no protection at all! I could pass a butter knife right through it, let alone see it turn a blade."
>Celestia looks genuinely upset at the quality of the bargain-bin royal guard costume Surfer had picked up on his way home from work last week.
>In his opinion it looked alright, but Surfer hadn't sat on the throne for two-thousand years so who was he to judge.
>The actor stands naked, still trembling, as Celestia scrutinises the armour before dumping it to one side.
>"A matter for another time, though. Now, I wanted to have a word with you about something personal."
>The actor, what was his name? Surfer had no idea. Female actors were the important ones, they were the ones whose name you had to remember, the males came and went like the weather.
>"Anything you need, Princess," he says. "I'll -serve- you in any way I can."
>He winks at the camera. Why the fuck did he wink at the camera.
>Celestia covers a giggle with her hoof.
>"Oh yes, I'm certain you'll -thrust- yourself into your duties," she says with her own obvious wink.
>The actor breaks out into a goofy, toothy smile.
>"I don't care how -shlong- it takes," he says, "I'll -beat off- any obstacles and -lay- your concerns to rest."
>Celestia yelps an "Oh!" before devolving into fillyish tittering.
>The camerastallion starts snickering along with them.
>Cloud Surfer slouches in his seat and rocks his head back to stare at the ceiling as his two actors decide they want to just make puns and laugh instead of having sex for money.
"Why is this so hard," he whispers to no one.
>A moment later, a small smile tugs at the corners of his mouth.
"Heh. -Hard-."

---

>The stallion is a brute. Regolith is his name, even Cloud Surfer knows that. He's notorious in the young industry.
>Celestia looks him up and down. Mostly down. She's a very tall pony and as big as Regolith is, she's bigger.
>Regolith only seems to realise this after he's closed the distance and come face to face with her. He peers up and seems to lose some of his swagger.
>"Hello, my little pony," Celestia says. She probably meant it with full sincerity, but Regolith shuffles his hooves and straightens his back and spine as much as he can.
>Cloud's eyes shift to the stallion's hooves, and he smirks.
>Regolith is very slightly standing on his hoof-tips in an effort to seem bigger.
>It's still not enough. Celestia's smile widens.
>"I look forward to working with you!"
>"Uh, yeah, you too," Regolith grunts.
>"And what will I be doing today, Cloud Surfer?"
"Uhh," Cloud has been thinking about that question all morning.
>He knows what she'll be doing, he just doesn't know how to phrase it.
>Ordinarily he'd just come out with it, but with Celestia he feels like he has to wear gloves.
"You'll be giving him a... blow... job?"
>Celestia blinks.
"You know what that is, right."
>She frowns and tilts her head.
>"Blow job... ah, I think I understand. So I'll be playing a hair-dresser, yes? Regolith here is my client, and he comes in for a manecut. I do what I can, and put his head in a blow-dryer at the end. Then I suppose somewhere along the way we'll have sex?"
>Regolith nods as if that's a completely plausible scenario.
>Regolith definitely wasn't hired for his intelligence.
"Alright, look, we'll take it one step at a time. Reg? Get on the sofa, we're gonna do a practice run."
>Reg slumps onto a nearby sofa and slouches with his hind legs spread. The sofa sags beneath his weight.
>Celestia perches next to him, hooves on her lap, back straight, face attentive.
"No, you'll need to be on your knees in front of him, Princess."
>"Ah, my apologies."
>She takes her position between Reg's legs. Even this low down, she's still on eye-level with the stallion.
>Regolith sits up, again as much as he possibly can, and for the briefest of moments he can see the top of Celestia's head.
>A self-assured smile stretches across his face.
>Finally, he's probably thinking, I'm the biggest in the room again.
>Cloud Surfer pulls a chair over and sits on it reversed, resting his hooves over its backrest.
"Alright, so pick up his cock."
>Like a true professional, Reg is already rock hard. His meat is thick, veiny, terrifying. Many mares, even other professionals, have balked at it.
>Many have flat-out refused to work with him for fear of pain.
>Celestia pokes it and giggles when it twitches.
>"You've got a very nice penis, Regolith. I like the colour. It's quite cute when it twitches! Like a little bouncing sausage."
>Regolith's self-confidence disintegrates and he slumps back in his seat until he can no longer see the top of her head.
>Killed him with kindness. She's ruthless.
"So you're gonna wanna... you know. Put it in your mouth."
>Celestia gives him a surprised look.
>"Really? That's novel. Is this a new trend?"
"It's... well, it's a blowjob. Surely you've heard of them. You -must- have done."
>Her expression is blank. Of course it is. She's as pure as freshly fallen snow. A heavenly angel made manifest on this wretched planet. Perfection given form.
>And now she's on her knees about to suck a fat cock. Cloud Surfer feels something like guilt for the first time in his life.
>He doesn't like it.
"Just put the tip in--"
>Celestia opens her maw and inhales the entire thing.
>Cloud stares in open-mouthed shock.
>Regolith is stunned.
>His angry, pulsating shaft, which ordinarily strikes fear into the hearts of mares, sits quite comfortably in Celestia's throat. Her lips are pressed to his crotch, and Regolith looks like he's having a religious experience.
>It throbs, clearly thrilled to find itself somewhere so accommodating.
>Reg shakily sits up a bit, casting worried looks between his boss and his co-star.
>"Lah iff?" Celestia says over a mouthful of fuckstick.
"Uh. Yeah. Holy Tartarus. Now you need to you suck on it, if you can manage, I mean."
>"Ah faww ihh wah a wow hob," she muffles.
"Yeah, it's called a blowjob, but it involves sucking."
>Her brow creases.
>"Weaw ah haw-he hink hahs ha hug hane haw ih."
"No, it's not a good name for it, but that's what it's called. There's some fancy name for it beginning with F, but I can't remember what it is."
>"Fuckin'?" Regolith offers.
>Celestia smacks his leg with a sharp hoof and he flinches.
>"Hanguage," she says sternly - somehow.
"Well, anyway, now you need to suck it."
>"Ohay."
>She remains completely still.
>Seconds pass. Reg scratches his head and gives his boss another confused look.
>He might not really be confused, Cloud Surfer is pretty sure that's just Reg's default expression. Mild perturbation.
"So are you..."
>"Uh. She's suckin', boss," says Reg.
"What."
>"I mean, like, just suckin'. Like a baby with a bottle, or somethin'."
"Oh. No, Celestia, you need to move your head back and forth, like this," he does his best impression of a pigeon walking around and bobbing its head.
>Come to think of it, how -would- you describe a blowjob?
"Just sort of... no, alright, use your tongue, lick your tongue up and down it."
>Celestia pulls her head back and releases Reg's sodden, glistening cock from the confines of her throat.
>It twitches sadly, pointing back up at Celestia as if to say "Hey, put me back."
>The Princess of Equestria takes the aching shaft in her hoof and licks it like a filly would a lollipop. Small, chaste licks.
>"Like this?"
"No, that's..."
>Cloud retreats into himself, rubbing his chin and trying to fathom how to explain to a millennia-old alicorn how she should suck a dick.
>"I dunno, boss," Reg says after a few seconds of Celestia's licks, "I think it feels kinda' nice."
>"Why, thank you, Regolith! I'm glad you're enjoying it."
>"I ain't never had a mare take the whole thing at once."
>"Oh? And how does that make you feel?"
>"It, um. It's whatever," he shrugs.
>Celestia frowns, her hoof still hooked around the shaft to keep it steady.
>"You look doubtful. What's the matter?"
>"Well, you know. It's big. Other mares get scared of it and... you know."
>"Are they afraid to be intimate with you?"
>"Yeah..."
>"And do you not like that?"
>Reg pulls his chin down, staring at his chest.
>"...No," he says quietly.
>"Do you not like mares being frightened of you?"
>"I ain't never had anyone like me for... me. It's always about my dick. Either mares are too scared to do anythin' with it and make excuses not to see me anymore, or they think I'm some kinda freak to check off a bucket list..."
>"Oh, Regolith, you're not a freak at all, don't be so silly."
>She drops his cock, gets up on the sofa beside him, and puts a hoof on his shoulders.
>"Would you like to talk about it?"
>Cloud Surfer re-emerges from his mind-palace.
"I've got it, so you--"
>He blinks.
>Regolith's head is on Celestia's lap and he's blubbering like a baby. What's worse, his mammoth bitch-breaker has retreated back into its sheath.
"Celestia damn it, Celestia."
>She gives him an apologetic shrug, then goes back to providing impromptu counselling.

---

>Cloud Surfer can't hide his grin, nor would he want to.
>He's actually managed to muddle his way into shooting an actual porno with an actual Princess.
>Princess Celestia, the star of the show, is riding a dick before his very eyes, and whilst the cameras are -rolling-.
>There haven't been any interruptions.
>No sobbing co-stars, no foalish giggling or inability to perform the most basic sex acts that even a high-schooler knows about.
>It's taken a few days, but he, and he alone amongst his peers as pioneering directors of this brave new industry, has managed to get Princess Celestia to fuck on camera.
>She looks like she's enjoying herself, too, which he's happier than he thought he'd be to think.
>Something about her smile casts light into the darker reaches of his degenerate soul.
>Perhaps she could redeem him. Use her limitless compassion and saintly nature to turn him from the path of darkness.
>Then again, she -did- come to him, and he -did- make her perform depraved acts for money.
>That thought stops him.
>-Has- she asked for money?
>So far she's just been showing up to work each day. There hasn't really been any sort of formal arrangement, he's just been rolling with it, like she's doing him a favour.
>Is she still running the country? Are the engines of bureaucracy still chugging along without her? Is her sister taking care of everything?
>Cloud Surfer hadn't put much thought into the systems of government before, but he was sure Celestia, as the goddess-queen of the land, must have some important role to play.
>That was before she started taking it to the hilt on camera though, so perhaps she got ousted in a coup. He'll have to check the papers.
>He isn't overly bothered. As long as he's able to milk this as hard as possible.
>And in regards to milking, Celestia's picking up the pace.
>Her coat ripples with each motion, taunt muscles clearly defined under the studio lights and coated in sweat.
>She's panting, which is new, and the stallion below her is openly moaning. Cloud doesn't think anyone's even acting at this point, which is perfect.
>The more natural it looks, the better, in his opinion.
>Celestia's flanks rise and fall like a blacksmith's hammer, each rise preceding a crushing blow.
>The bed quakes with each strike, the sound of strained wood barely heard over the thrashing bodies.
>Cloud Surfer's smile fades somewhat and he motions for his assistant to get the actors' attentions.
>A silent cue, done with a hoof gesture, will signal them to change to a new position without a clunky cut. The fewer obvious transitions, the better.
>The assistant waves off-set. She waggles her hoof in the air, then shoots Cloud a worried look.
"Celestia!" he hisses.
>The Princess isn't listening. She's got a strange expression about her, smashing her flanks against the stallion beneath her with abandon.
>Heavy, laboured pants launch flecks of spittle down at the stallion's face.
>The stallion pats Celestia's front leg with a hoof as one of their pre-arranged safety signs.
>She doesn't stop.
>He forgets subtlety and starts patting her leg repeatedly.
"Celestia!" Cloud shouts.
>The Princess pauses for half a second. An imperceptible halt in the frenzy.
>She lifts herself off the cock until the flared tip stands tall and pressed to her sex.
>Then collapses down on it with such staggering force that the bed's four legs spray out and snap off, the whole mattress dropping to the floor with a powerful 'whump' that sends a blast of musky air in all directions.
>The script is carried away off the stand by the director's chair, but Cloud doesn't notice. He's staring in mute shock at what he's seeing.
>After a second or two of silence, a low, anguished wail comes from beneath Celestia.
>She blinks, like coming out of a trance, and looks at the face of her colleague, then further down at place where their crotches are firmly pressed together.
>She catches Cloud Surfer's eye and forces her biggest, cheesiest smile.
>"Whoops?"

*

>The stallion is carried onto the back of the hospital wagon by the paramedics, and the wagon speeds off down the road.
>Celestia stands beside Cloud Surfer with a wounded look, like a child caught in a lie. He didn't think it was possible for a mare of such stature and godlike prestige to look small, but stranger things have happened in Equestria.
>"Again, I cannot--"
"Please don't."
>"I can't imagine how upset you must be--"
"For fuck's sake," Cloud massages his temple.
>"I unreservedly apologise, I'll make it up to Chip, the studio and yourself, I promise."
>Cloud turns away from the street and trudges back inside, staring into space as terrible legal scenarios play out in his mind.
"You broke his fucking pelvis. The insurance isn't gonna cover this one, oh Tartarus we'd better not get investigated for this. He's gonna sue. I -know- he's gonna sue."
>"The Crown will cover all expenses, and we'll make this problem go away, Cloud Surfer, -please- look at me, I'm sorry!"
>To her credit - and because of course she does, she's Celestia - she looks distraught.
>"I got carried away. It's been a long time since I've had sex that hit the spot like that, it's quite rare for another stallion to last that long, you must understand that as a mare..." she trails off when she sees Cloud's expression.
"You broke. His fucking. -Pelvis-."
>Her ears flatten against her head, and she doesn't correct his swearing.
>"I... yes. I did. I apologise. Sometimes I forget my own strength. And weight. Do you think Chip would appreciate flowers? I'll get him chocolates too, I'll say they're from the studio. Will you visit him in the hospital with me?"
>Cloud Surfer slumps into his director's chair and casts a weary look at his camerastallion.
>The stallion glances at Celestia, then takes two nervous steps away from her, as if he was at risk of being crushed under her titanic flanks as well.
"Did we get some good footage out of this, at least?" Cloud grunts.
>"About five or so minutes, boss."
"Well alright, this wasn't a complete loss then. Do we have any backups?"
>"We did, but..."
>Both Cloud and the stallion look at the bench at the back of the room where one or two back-up stallions would normally sit in case they were needed for a scene.
>The bench is empty, and the back door to the studio is wide open.
"Son of a bitch, Celestia," Cloud grumbles.