Mark Millar still has the air of a man who needs to pinch himself to appreciate his good fortune. After 20 years of rupturing the boundaries of taste and decency, the enfant terrible of the comic world has finally hit paydirt. Millar's stellar levels of bad language and violence may not raise eyebrows in the geeky world of comic novels, but when transferred to the big screen in Kick-Ass, they have generated the sort of publicity certain to ensure the film's success.

"Anything goes in the comic world, so sometimes you forget the sensibilities of the mainstream," says Millar. "But even I chuckled to myself at that line, because I knew it would cause a huge amount of fuss if it ever hit the big wide world. Sure enough . . ."

He is referring to the line delivered by Chloë Grace Moretz, aged 13, who plays 11-year-old child vigilante Hit-Girl in the Glaswegian's latest superhero offering. "OK, you cunts, let's see what you can do now!" The fact that the c-word was suggested by Moretz's mother, on set to help her daughter through filming, has done little to mitigate the moral outrage – which was further stoked by the presence of Jonathan Ross's wife, Jane Goldman, on the screenwriting credits.

"Look," says Millar, "we tried everything – we even came over all British and tried, 'OK, you wanker.' Nothing had the force of the line that we used; it was just right, it worked."

Millar, himself a father of young girls, is as amused as he is unrepentant. The film, he says, would be a roaring success even without the line. "After the post-premiere party, [director] Matthew Vaughn and I were congratulating ourselves, and then I stopped and said, 'We haven't sold a single ticket yet.' But in a funny way that doesn't matter – I always knew Kick-Ass would be a successful film, even when it was being turned down by studio after studio and we were being told it was rubbish."

On the page, Millar's outlandish plotlines have made him the art form's most powerful influence since Spider-Man's legendary co-creator Stan Lee. In the flesh, this garrulous lay preacher is explaining how he squares his devout Catholic faith with his fascination for gory violence one minute, and the next musing on working-class Glasgow's love affair with Americana, or his reaction when a recent bout of flu was misdiagnosed as cancer.

But mostly, Marvel Comics's chief writer talks with wide-eyed wonderment about celebrity. Hollywood's rediscovery of the power of comic books has propelled him to the apex of popular culture – beginning in 2008 with Wanted, a larcenously amoral tale starring Angelina Jolie, Morgan Freeman and James McAvoy, which took $350m (£235m) at the box office.

Millar's steep upward trajectory has continued this year with Kick-Ass, which he describes as "a hymn to neo-conservatism", and he says there's plenty more to come. He is, though, determined to get out before he is 45, the point at which he believes the creative juices dry up.

"I had no misgivings about making the jump from comics to the mainstream," he says. "Lots of people in comics want it to be their little world, where only people who are genuinely into it are allowed. But I've never got what's cool about nobody looking at your stuff. The only people with an interest in being snooty about the mainstream are those people who can't reach it."

Hailing from Coatbridge, one of Scotland's most deprived communities, the fact that Millar gets to accompany unlikely architecture nerd Brad Pitt on his annual tour of Glasgow in homage to Charles Rennie Mackintosh is a source of wonder to the writer. So too was his recent appearance at the Oscars, where he found himself courted by A-list stars keen to take parts in his future films.

"One of my pals is absolutely average-looking but he always seemed to have absolutely beautiful girlfriends," Millar says. "He said that if it becomes known that you have really good-looking girlfriends, then you attract good-looking girls. It's like a club, and it's the same with the movies: Angelina really liked Wanted, so Brad wanted to come on board to produce Kick-Ass."

Not that the process of transforming Millar's graphic novels into celluloid creations has been easy. Initially, he was just happy to be involved, but then he looked at the cast for Wanted. He had never heard of McAvoy or Kazakh director Timur Bekmambetov, and "had just seen Borat, so presumed Bekmambetov must be cheap Russian labour". He looked at the credits of the producers and screenwriters – "not just The Fast And The Furious but the crap sequel 2 Fast 2 Furious, plus Legally Blonde" – and freaked out. "I thought it was going to be totally, embarrassingly rubbish," he laughs. "Fortunately, they conspired to make it brilliant."

Intoxicated, Millar thought he'd take a lead role in Kick-Ass. "I was executive producer of Wanted, which meant being involved in a couple of phone calls – 'Q: Do you want Angelina Jolie involved? A: Yes' – and I got paid a big chunk of cash for that. But with Kick-Ass, I've been shocked at how much work is involved. I was in on all of the casting: we looked at hundreds of people. Then there was drafting the script, the costumes, the sets and the filming – 14 weeks, doing 14-hour days."

The script was a particular problem. Millar, used to autonomy, was shocked at the accommodations he had to make. "I don't really do happy endings, so there's a huge difference between Kick-Ass the movie and Kick-Ass the comic," he says. "In the movie Matthew [Vaughn] really wanted the lead to get the girl, whereas in the comic, this guy is a loser and pretends to be gay because the girl works in a shelter and is really right-on. She just wants to be best friends, he wants to have sex with her, and in the comic when he confesses she tells him to fuck off. In the movie, Matthew has them having sex."

Like virtually every storyline in Millar's career, the baseline for Kick-Ass comes from his own experience. He's never pretended to be gay, but as a teenager he did convince a girl he fancied that he was as obsessed with Dynasty as she was, studying videos of the Colbys to complete the facade. Millar is convinced his success is down to his upbringing: his mother died when he was 14 and his father four years later, so he had to drop out of university to bring up his brother. Money was so tight that "the cat ate one day and we ate the next". Writing was an economic imperative.

Glasgow also gave him the cultural antennae to be successful. The city is built on the same grid system as New York ("there's more than an element of Gotham to Glasgow; it feels like a more modestly budgeted version of New York, only far more violent"), and west coast Scots have a fascination with the US that provokes a special bond: "I feel a kinship with American kids – I like what they like because it's what I like too. The collapse of the US economy is a godsend: what's terrible for the world is great for writing. The last eight years in particular have been good to me," he says, in a reference to George Bush.

There is a moral core to Millar's populism, as you'd expect from someone who cites Jesus and Tony Benn as heroes. The battle between good and evil are constant themes, as is redemption. But so too is an eye-watering level of violence. "It's cathartic," he says. "Besides, you need movement in a film, and to see superheroes pounding each other or picking up a car and whacking someone with it is visually exciting."

Although he has thought about moving to the US, Millar remains rooted in Coatbridge, the "Little Vatican" that has shaped him. Now in his last year as Marvel's chief writer, he has commissioned a largely Scottish group of friends and comic aficionados – Frankie Boyle, Muriel Gray, Ian Rankin, Armando Iannucci, Jonathan Ross and Russell Davies – to pen graphic novels for him.

"Glasgow's the perfect education – it's given me a unique life experience compared to everyone in the New York publishing industry and Hollywood. Every single person in Hollywood looks the same: the writers are all skinny, bald guys with glasses, who hang out in coffee shops all day."

• Kick-Ass is out on general release on Wednesday 31 March.