Mark Millar's graphic novels really are graphic but the Coatbridge boy behind Wanted and new teen film Kick-Ass is suprisingly mild-mannered

So you might find Mark Millar stretched out on a bed in Angelina Jolie's trailer watching DVDs with the controversial sex bomb during the filming of Wanted, the movie version of his ultra-violent bestselling comic, which has grossed more than half a billion dollars and whose poster, featuring Jolie wielding heavy duty firearms, caused ructions around the world.

Then again, you might find him chewing the fat with his boyhood mates over pints in a Glasgow pub. Mark Millar, the author of violent, expletive-laden comics is also Mark Millar, practising Catholic, devoted father, and a man who never – ever – curses.

Cheekily admitting that he adores talking about himself, he also delights in causing controversy. Just as well - there'll be plenty of it when the film of Kick-Ass, his take on the Spiderman myth, reaches cinemas on April 16. He's on record saying that it will "make Wanted look like The Prime of Miss Jean Brodie."

Already a popular comic, and due out as a hardcover collection next March, Kick-Ass is set in the US and chronicles the adventures of a motherless teenager, Dave Lizewski, who's portrayed by Aaron Johnson in the film. Dave confects a superhero costume and takes to the streets to battle baddies. His first time out he's beaten comatose, but undaunted, returns to the streets once his bones have set. On his rounds he meets a ferocious young assassin, Hit Girl – played by Chloe Moretz – working in tandem with her father, Big Daddy, portrayed by Nicholas Cage.

"We'll probably have to leave Britain when the movie comes out," jokes Millar. "I think it's going to cause a storm, these children stabbing each other, using machine guns. People see a little girl and the superhero costume and get lulled into a false sense that nothing bad can happen, but then she kills people and curses a blue streak!"

As on Wanted, Millar took a producer's role, overseeing every aspect, including the auditions. Having blocked out the storyline, he turned screenwriting duties over to Jane Goldman – aka Mrs Jonathan Ross, whom he knew socially, since Wossie shares his passion for comics.

"Jane Goldman is literally the best screenwriter I've ever seen. And she looks like a super heroine! Jane has given the story a level of depth and humour and wit that the original didn't have."

Casting proved more problematic. "You can't find real teenagers who are charismatic. The whole movie hinges on how good this guy is. He's got to be a geek and you've got to care about him. In the final week Aaron Johnson, who was in Angus, Thongs and Perfect Snogging, came in. We'd said we are definitely not casting a Brit , because we didn't believe a teenager could do the accent convincingly. But he pretended to be an American. We didn't know (he wasn't] until the third call back.

"But Chloe Moretz, has stolen the movie. When she came in it was like the same feeling they must have had when Jodie Foster went in for Taxi Driver. We couldn't believe how convincing she was.

"I was on the train from London and phoned my wife, who asked how did it go? I said 'Amazing!' And remember, this is from everyone else on the train's perspective, and I am saying, 'I've just been watching lots of DVDs with lots of little girls over the past few days down at Matthew's place and there's one in particular who I just love. She's 11 but she can play 9. And her mum said that she's up for anything' – meaning, of course, that she can swear and things like that – 'and anything she can't do we can get a midget to double up for, anyway.' I am suddenly aware of every pair of eyes on the train looking at me. I was expecting the transport police to be waiting on the other end."

Matthew, of course, is the film's director, Matthew Vaughn, who came to fame with Lock, Stock, and Two Smoking Barrels and as the consort of Claudia Schiffer. He had the ingenious idea to finance Kick-Ass independently and then auction the distribution rights, which were snapped up by Lionsgate for an estimated $50 million.

It sounds breathtakingly glitzy, but Millar's decidedly un-starry back story bears has been re-created to an extent for Lizewski. "Everything that happens to (him] happened to me, although it's slightly skewed," he has said. "The stuff that's absolutely pathetic is the stuff that's real, like Larry David put a lot of himself into George Costanza."

Millar was a change of life baby, the last of six – four brothers and one tomboy sister – born to a cleaning lady and a labourer. Growing up on a Coatbridge council estate, he hadn't a clue there was such a thing as a "lifestyle".

"You'd see Blake Carrington on television but you thought it was fiction. You didn't know this kind of incredible wealth really existed. Nobody on my street had a car; we were all about the same, so you didn't feel underprivileged."

His siblings are 22, 20, 18, 16 and 14 years older, so it was like having droves of parents, all of whom doted on him."Every single one would take me places, so I was quite spoiled. They were all coming out of uni and getting jobs and doing OK, and would say, do you want to go to the cinema this weekend, do you want to go bowling? I got indulged a lot."

Asking about his first comic elicits a mega-watt smile. "I remember it exactly. I was 4 . My brother Bobby was at university. He'd take me into the shops and say, 'Oh yeah, that's the one you wanted,' and really buy them for himself! I would get them after. One was Spiderman, probably the most famous issue that has ever been, when his girlfriend dies, Gwen Stacey. This issue is momumental for fans and it's the first one I ever read, when I was still just figuring out words. And there was a Superman comic, too. I discovered Superman and Spiderman on the same day."

Millar was soon dressing up like his favourite characters, and evinces regret at the loss of a photo of his first communion. In it, one can see vestiges of a spider web he'd drawn across his face a week earlier with indelible marker. His mum scrubbed and scrubbed, but the evidence lingered.

"I also made an Iron Man suit out of paper, coloured it all in and then stapled it all round myself. And I had a Superman suit that I wore under my school clothes when I was about seven. It was plastic. By lunchtime I could hardly breathe and the teacher opened up the windows. Sweat was pouring down my face. Eventually she started undoing my buttons and took my shirt off and it was like my secret identity exposed. I was humiliated!" He laughs. "My whole childhood was littered with stuff like that."

One morning when he was 14, Millar was with his 64 year old mum when she suffered a fatal heart attack. Shocking as it was, he felt a sense of inevitability about her death.

"When I was about nine I remember suddenly being aware (that] my pal's mums were a lot younger. I realised mine was a good 15, 20 years older, and I thought, 'that's a bit wierd.' And then I thought, 'Oh my god, she'll probably die before the other mums.' I did a little arithmetic – most people in my family tend to go in their sixties, that whole west of Scotland thing – and I thought she's going to die while I'm a teenager! I remember bursting into tears."

He dried his eyes and stiffened his upper lip, then promptly ran into his dad, who asked if everything was OK. "I said I've just got something in my eye. I didn't want to freak him out by telling him he was going to die! On some level, yeah, it armours you up a bit, prepares you for it. At the time I didn't think she was that young. Dad was a little younger than her, and died four years later at 65. But they seemed like old people by then."

The household now consisted of Millar and his dad, who'd never cooked a meal in his life. "We had the same meal every night for four years! Minced beef. I remember after about three hundred dinners I said 'Dad can we try something else', and he said 'I don't know how to make anything else.' He went to the supermarket and bought a pie . . . that was mince inside! Sometimes I'd go to my brothers' places and get something else to eat there."

Didn't he think to bring home the recipes? "That's the thing, I was always complaining but I didn't really do anything about it myself. I should have been cooking."

Millar went to Glasgow University to study politics and economics, but dropped out after his father died and the money for essential expenses dried up. When he was 17 he began dating Gill, who lived nearby and attended the same school. They married in 1993, but in the spring of 2009, the couple separated amicably.

After years of knocking about, his career took off properly in 2000. But when did he realise that writing comics could be an actual job? "Immediately. I knew you got paid a certain amount for each page, no matter how many words it contained. The exact same wage. So sometimes, yeah, you drop a few Kapows into the mix! Scotland's economy was down the tubes. There were no jobs. So I thought it was this or nothing."

He's worked for the major players – DC and Marvel – but discovered that the real money, not to mention artistic freedom, comes via "creator-owned" work, meaning characters he owns outright, which he can write for, or license out. Cases in point are Wanted and Kick-Ass.

I'm intrigued that Millar, who's drawn all his life, deliberately doesn't illustrate his own stories. "When I sell a movie I always give half the money to the artist who did the original book. My agent goes nuts. Everything, all the money from the advance, the toys, the games, 50-50, so it's fair. I see it as a collaboration. If I drew, I could keep everything for myself, but I actually like the idea of having a talent that I'm not selling. One of my friends said, 'You could make even more money by drawing, too,' and I said, 'Yeah, and I suppose I could make even more money by lap dancing. It's nice to have something that's just for me."

I cannot reconcile this charming pussycat of a man with his blood-drenched, violent comics. "I was talking to Mel Gibson about this," he says, giggling at the clunk of a name being dropped. "About The Passion, which was called the pornography of violence. It didn't seem violent to us because we grew up with the stations of the cross. Buddhism is a fat guy sitting around smiling, but we get a guy nailed to a piece of wood, wearing a crown of thorns, screaming in agony. That's your first image as a child in a Catholic school, and the weird thing is you don't really notice. Then in the stations of the cross, you see him get beaten. And we also both grew up in tough places. So maybe we internalised the violence and it comes out in the work.

"But I've also found that people who do really dark, really violent work tend to be quite well balanced and the people who do really light stuff tend to be really weird guys! Maybe it's a kind of catharsis."

Finally, what makes a hero – or heroine? "It's trying to do the right thing against terrible odds. Kick-Ass is all about a 16 year old guy who hasn't got a lot of resources. He's a little guy who went on Ebay and bought some stuff and he's got two sticks and he's trying to help people. There's something so sweet about that, even though what happens to him is horrible. To me, heroism comes from him going back out there every night, even though he's getting really badly beaten up."

Never one to shirk from an artistic challenge, Millar next plans on "scaring the hell out of himself" by writing and directing a film. For now the project's shrouded in secrecy. Let's hope it's not too long before he rips open his shirt to reveal his newest incarnation.

Kick-Ass #8 is published on 27 January, by Marvel; Kick-Ass: The Graphic Novel is out on 1 March 2010, 14.99; Kick-Ass the film is released in the UK on 16 April.