Even when he's not speaking - usually in the gap between being asked a question and starting to answer it - James McAvoy is never quiet.

Listen closely and he is constantly making a noise. Most times he'll be clicking his tongue or smacking his lips. A noise to forestall words. At one point he even essays a version of Deep Purple's Smoke on the Water (you know the one; it goes "eh eh eh, eh eh eh eh" when McAvoy does it), while weighing up what he's going to say.

In short he's filling the space between us. He's good at filling spaces. He's spent the last decade doing just that on our screens, both big and small. He's about to do it again, in the film version of Irvine Welsh's Filth; a love-it-or-hate-it black-humoured slice of furious misanthropy (with a side order of misogyny) in which he excels as a bent, nasty, abusive detective sergeant and Hearts fan called Bruce Robertson.

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Filth is the (rather bruised) cherry on the cake of a year that's seen him appear in Danny Boyle's wild, thrilling mess of a movie Trance and the inevitable London gangster movie Welcome to the Punch, as well as playing Macbeth on the London stage. A few days after we meet he's off to Toronto to play Professor Xavier again in the latest X-Men movie.

Today, though, it's early summer in north London and we're sitting on a balcony of a studio with a backdrop of the city skyline shimmering behind us, discussing mental illness, photographing intimate parts of the body and how serious he was as a teenager about becoming a priest.

It's been the best part of 10 years since I last spoke to McAvoy. Then he was a fresh face, beginning to get himself recognised thanks to his parts in two Paul Abbott TV dramas, State of Play and Shameless (on which he met his wife Anne-Marie Duff). Since then he's starred opposite Angelina Jolie in Wanted, Keira Knightley in Atonement and Anne Hathaway in Becoming Jane (Forest Whitaker in The Last King of Scotland too) and generally built himself a career that he can't have imagined as a kid in Drumchapel. Or even as the man whom I met nearly 10 years ago. What was his ambition back then?

"Well, your ambition is to do just as well as you can, isn't it," he says. "If you were to ask me now 'what's your dream part' I couldn't tell you, mate. And I think I was the same then. My ambition was to take the next job that was put in front of me. Always.

"I couldn't really go up for a part - even if I didn't like it, even if I thought it was pish - and not want to get it. That's the only competitive streak in me as an actor. If I start working on a job with actors I don't get competitive with them. I just want to play my part. If I'm in an audition I really want to get it. So I try really hard and that helped me a lot, I think."

He pauses, recalls the original question. "My ambition was to keep working, pay the bills and maybe own my own house one day. I do that now. A particularly Thatcherite ambition, isn't it? I am a child of Thatcher. I was born in 1979, the year she came in, and for 12 years I flourished in her shadow."

I think you can take that latter statement with more than a pinch of irony. As an interviewee McAvoy is a curious mixture of full-on seriousness and offhand humour. He is very earnest about what he does. Filth - which he helped produce and which is one of the better adaptations of Irvine Welsh's books - is a very black, very filthy, very funny take on a serious subject. Watching it you can tell that McAvoy is having fun. But ask him about it and instead he wants to talk about the thin line between sanity and insanity. "The character, I felt, was a great opportunity to investigate and portray mental illness in a really exciting, fun and ... uh ... odd way. There's a lot of sadness in the film but my experience of mental illness is not just that it's sad all the time. It's manic. That means high energy and can quite often be hilarious as well as deeply moving. And harrowing.

"And I thought, 'yeah, I know how to do him. I know what to do with that character.' And that's when you get excited: 'I'll do that with that bit and I'll do this with that bit and I'll tone that down there, but I'll bring that up and then I'll make it more about that there and then I'll try to make them laugh there because what's going to happen next is I'm going to make them f****** cry with that bit hopefully'."

Just from that you can tell the pleasure in the role for McAvoy. It's in the sense of play and a playful approach to sensitive material. The book, he says, is really about the physical degradation of Robbo. The film he hopes is more about the existential. "It's not about his flaky balls, it's about his flaky brain."

He expects the film will offend some. "We are going to have people who will leave the cinema, but we knew we were setting out to make a film that, should we make it properly, people would either love or loathe."

Let's rewind a little here. About mental illness. What is the personal experience he mentioned? Is it in his own life? "Not my family, no. I've known lots of people with quite significant mental issues."

It's more common than we think, he reckons. "I believe every single person is only a couple of steps away from mental illness. A lot is borne out of the fact that we don't accept the world the way it is so therefore we create an alternative. Or we create a delusion that we'd rather believe.

"We've all been in that position where we want the world to be slightly different and it's when we want it so hard that we change reality a little bit - that's quite interesting to me."

He could be talking about acting here, I point out. "Yeah. And I think that's why I've enjoyed playing it so much this past year. [The character] in Trance had psychological trauma done to him and Macbeth is a f****** nutjob - the way I portrayed him anyway. I do think actors are quite close simply because actors are quite self-aware. I think we're all close to it, but as an actor I think I've been aware of when I've only been a step away from it rather than the usual two steps. And you go 'whoah, that's interesting'. Don't get me wrong, I'm not method in any way. But there are times when you get lost in it a wee bit and that must be what being mentally ill is like. We get glimpses of it."

What are the overlaps between James McAvoy and Robbo, I wonder? Let's test them. Have you ever broken the law? "I've flown the Stuart coat of arms. That's breaking the law. You're not supposed to do that at Scotland matches because the Windsors are the royal family. But I think I'm fairly law-abiding."

Have you photocopied your wedding tackle? "No. I took a picture of it when I was 16." When was the last time you were in a fight? "A guy tried to take a picture of my d*** when I was having a pee once and that turned into a fight. But he had a lot of mates and I had a lot of mates so it turned into a big ruck. It was quite hilarious.

"The last fight I got into was eight years ago on a night bus going to Stratford where I used to live in the East End." He was with his friend Tom Ellis, best known as Dr Cousins in EastEnders, at the time. "And I've always remembered Tom getting hit and doing the Alan Partridge line: 'Awww, that really hurt'. And I just thought 'he's f****** committed to one-liners. Even in a fight'."

It's very clear that McAvoy likes his job. Hard then to imagine the alternative lives he imagined for himself when he was a teenager in Glasgow in the mid-1990s. The only options, he could come up with at the time, were to be a priest or join the navy.

"My idea to be a priest was short-lived. It was when I was 16. And it wasn't just any old priest. I thought about being a missionary. That was the only kind of priest I'd like to be, so I could see the world. That was the real reason. I thought I'd get to exotic places and help people. And then I realised 'yeah, but I'll have to get them to lose their beliefs and believe in my beliefs. Oh God, I don't even know if I believe ... And also girls have got nice boobs'."

He had the qualifications to join the navy's officer training school and his mate was joining so he thought seriously about it but then he'd got a small part in a movie - The Near Room - when he was 15 and thought he should try out for drama school. He applied for the Royal School of Music and Drama in Glasgow (as it then was) and "thankfully I got in, because I've had a better life out of acting than I could have dreamed of, man".

Well, indeed. It gave him a career that has taken him around the world - the thing he wanted from the priesthood and the navy - choice roles and the chance to kiss some of the biggest names in Hollywood. Who was the best at it? "My wife." Is there a male actor you'd be prepared to snog? "My favourite actor - so if I get to work with him in a movie and the price is that I need to give him a snog - is Sam Rockwell. My favourite living actor."

If you have a sturdy constitution Filth could make McAvoy yours. He is the best thing in the film by a mile (and that's not a criticism of the rest of the movie). And it gave him the chance to play Scottish again. You could say this has been his year of being Scottish, what with Filth and his nearly all-Scottish cast of Macbeth. "There were a couple of members of the cast who weren't Scottish. Claire Foy was one and she said she found it quite amazing working with us lot because 'you guys just say what you f****** think immediately'. I would never have characterised Scottish people as that and I don't necessarily believe in generalising nationality but I do think she had a point. What happens, she said, is 'you guys say really offensive shit but you apologise for it immediately.'

"That's probably true. My wife is of Irish stock and her uncle has always said Irish people are like 'yeah, f****** brilliant, brilliant, brilliant ... see the f****** thing about you ... Whereas Scottish people are 'you're a f****** c***, you're a f****** c*** ... Give me a cuddle! I love you.'"

I don't think we get that far today. James McAvoy has to go. He has a wife and a son - Brendan, born in 2010 - to see before he disappears off to Toronto in a few days.

If it came down to it, I ask, would he spend the time between then and now by watching one of his own movies? "If I've got two hours to myself of an evening - which is rare - I'd rather watch somebody else's film because I know what happens in mine. I can watch myself and I can be rational about it and not go 'oh God, I hate looking at myself'. I'm over that now. But I'd much rather watch somebody else's film than my own."

That's one space McAvoy doesn't feel obliged to fill. n

Filth, 18, opens in Scotland on September 27