Before superhero movies, directors were masters of the universe – now you can find them cowering in their trailers

When the wheels are coming off, there is no more exquisite humiliation that can be visited upon an adult human than being the director of a big-budget superhero franchise movie. Not even working as the guy who had to wipe a medieval king's arse. "Groom of the Stool" is sometimes a more covetable credit than "Directed by". And as even the most fearsome talent agent will tell you, both guarantee you get shit on the back end.

But that's confusing, you might think, because aren't directors supposed to be god tier? That's definitely what I thought, back when I started as one of the writers on The Franchise, a new HBO comedy set behind the scenes in the world of superhero movies. Except the more we talked to people inside the comic-book movie machines of Marvel and DC – and we talked to huge numbers of people – the more dysfunctional the picture that emerged became.

A director told us about the moment they realised they were being fobbed off with busywork shots of a door being opened, while a second unit was somewhere else with the lead actors, filming the big scenes the studio were actually going to put in the movie. Another told us about individual stars hiring individual writers to punch up their characters' lines – and punch down everyone else's. We heard about limos pulling up on set, the window going down, and new script pages for that day being passed out. Directors, those masters and mistresses of the universe, were surprisingly keen to relive these indignities. Their movies had become something that was done to them, not by them. They talked about the best survival strategy being to "go limp".

And the chaos! There were multiple times in the writers' room where we said: "Yeah, I know it really happened, but we can't actually put it in. People won't believe it. It's too stupid." The idea of starting shooting a $300m movie with no third act settled upon seemed insane, but was almost standard practice. Actors shot with amorphous green props that would be VFXed in later, either because no one could agree what they should be, or because the provisional wing of the fandom had threatened insurgency/homicide over their design slightly deviating from the comics. It turned out that, backstage, the movies that had become the dominant cultural product of our time were more chaotic than even cold war proxy conflicts. And frequently, less uplifting.

Plus, in a maybe-not-unconnected development, the cracks were opening up. The Marvel Cinematic Universe (MCU) – once the most consistently bankable franchise in the entire history of Hollywood film-making – was suddenly faltering, and even turning out a flop or two. DC's reboot was supposedly taking place even as studios were hearing about a potentially deadly new cultural pandemic – "superhero fatigue".

But wait, because I'm getting ahead of myself. Like a number of late-stage superhero franchise movies, I've started in the wrong place. So let's spin this off into the origin story ...

Picture the scene. You are the director of a superhero franchise movie. When the studio hired you with mega fanfare, they gave you a budget that could have bought one hundred thousand water wells across Somalia. Which, you know, you didn't like to brag about. But it was a pretty neat way of reminding yourself how important your movie was going to be. Then again, you loved cinema above all things, so what you did with your life was always going to be important. You grew up idolising all those guys: Marty, Francis, Quentin. Mister Spielberg. Age 17, you accepted you shared their destiny. Aged 19, you decided your autobiography was going to be called My Auteurbiography. Aged 21, girls had sex with you to stop you explaining Mulholland Drive to them for another hour. The next morning, all you had to say was: "I think you'll find that structurally, it's a Moebius strip ... " and they'd do it again.

Aged 26, you were fresh out of graduate film school and keen to establish yourself as an uncompromising maverick, who was also available to shoot fragrance commercials. Aged 27, you read the phrase "Sundance-to-spandex pipeline" about how promising young indie directors get sucked into the superhero sausage factory – and passed it off as your own in conversation on your first date with a model and award-winning brunch DJ who recently curated her own range of raw antidepressants.

Aged 31 you made Breakfast Serial, a low-budget satirical slasher movie about a diner chef hunting down unfavourable TripAdvisor reviewers. A thin-year festival hit, it played at Sundance, where you self-effacingly mentioned to the audience that critics were already calling it "a slow-burn funscape that's as deliberately undigestible as a human femur". Barry Diller sent peonies.

Also aged 31, you married the model and were formally designated a power couple. Sure, OK, Paper magazine?! For you, all that stuff was just noise that you weren't going to let get in the way of the work. Even so, you bought a $2,000 chore jacket. You started calling watches "timepieces".

And then, aged 32, the comic-book studio wanted you. They wanted you in the way that you had always wanted to be wanted, maybe even by your wife. They wanted you to take your time thinking about it but, obviously, they wanted to announce your movie at Comic-Con next week. You told reporters what an honour it was to put a new spin on a character whose stories defined your childhood. (Did they? It didn't matter. No one could possibly have any way of checking.) The studio said they loved your uncompromising vision, so you knew you could persuade them to make it darker and grittier. You knew you wouldn't get caught up in the machine.

Needle scratch. Cut to now. Now, aged 36, you are sitting in a trailer an hour before dawn on a location shoot in a quasi-democracy to which the studio has deployed you in order to take advantage of the tax credits. You are reading an in the Hollywood Reporter about how Christopher Nolan has total creative control over Current, his new movie about Nikolai Tesla. Before that, you read an about Yorgos Lanthimos developing an intensely complex and disquieting story about a power struggle within an uncontacted tribe. And you? Your movie is about a man who can grow his really strong hair really fast, fashioning it into makeshift weapons and life aids. This is very isolating for him, and also some people killed his parents when he was a kid.

Your movie was supposed to take nine months but you have now been on it for two-and-a-half years. It is in its second run of reshoots and has had more script revisions than Wikipedia. And possibly more writers. It's bad enough not having an ending but now they're rethinking the beginning. Thematically, it is both for and against the idea of war. You dimly recall it once had a climate message. The shoot has produced 10,000 tonnes of CO2 (so far).

Ninety-seven days into shooting you developed a nervous tic, a compulsive humming that frequently ruins takes. Or maybe improves them, it's hard to tell. The crew used to like you; then they hated you; then they pitied you. Your wife now lives with Chris Pine. You want to quit, but if you do, the studio will quietly tell the entire industry that you are difficult. But today your jailers are going to let you choose the name of a space parliament that they will later cut from the movie. Won't that be fun?

No, would seem to be the obvious answer. Later, when you nail on a smile for the premiere, you'll find what else they cut at the absolute last minute. As for the stuff they added ... your movie was cuckooed. The set of a vulnerable person – you – was moved into and taken over by studio forces who wanted to use it to advance their own enterprises. They pumped it full of last-minute product placement, random artefacts and non-sequitur scenes whose sole purpose is to tee up and serve characters and plotlines in other, infinitely more important movies in the universe. You come to realise that the movie you sold your soul for is a kind of indentured straight man. You have made a $250m content butler. Marketing will push it to $400m.

Your studio producers kept telling you that the directors who thrive in the franchise movie era are "the ones who know what they don't know". You finally understand that this was a nice way of saying your role was always more ceremonial than operational. You were only ever a foam-suited sports mascot, or a regimental goat. For now, hiding in your trailer, you accept your real place in this comic universe. You are both the most important and the least important cultural unit you have ever been. You are a superhero franchise movie director.

Or ... are you? And that's when the real twist hits you. That's when you get what's truly going on. That's when you realise you M Night Shyamalanned yourself. You suddenly understand that what you're actually making is another horror – a story where someone, in a really grotesque fashion, is killing your beloved cinema. And that someone? It can't be, can it? But it can. Oh my God, man – that someone is you! You killed cinema! IT WAS YOU ALL ALONG!

Roll credits. And no – you can't have your name taken off them.

The Franchise begins 21 October, Sky Comedy and Now.