So, here's today's question. Is James McAvoy as much of a cocky little bugger offscreen as he appears to be on? Actually, the ''little'' we can take for granted. In the flesh he is a slightly built thing, dressed this Edinburgh morning in what looks like an expensively anonymous cardigan and jeans, graced with a beard that's on the neat side of straggly.

But what about the cockiness? (We can probably forget the buggery element - he's not that kind of actor.) It is jejune, I know, to conflate the actor with the parts he plays but if you saw McAvoy as the sparky, full-of-himself young journo in the BBC's excellent political thriller State of Play last year, or catch him as the big-mouthed, wheelchair-bound Rory in the forthcoming Irish film Inside I'm Dancing, it's hard to imagine the actor is any different to the roles he inhabits. In both parts he is cocky to the max, even uberbrash.

As the spiky-haired, noseringed Rory in Inside I'm Dancing, he marks his arrival at Carrigmore residential house by telling everyone that he still has the use of two fingers, ''sufficient for self-propulsion and self-abuse''.

It seems only fair then to ask if there are elements of McAvoy himself in the lariness of both roles. So I do. ''No,'' he replies emphatically. And then starts to waver. ''Hmm, well yes,'' he continues, ''I do like pushing it a wee bit. I do find that kind of character quite interesting because I like getting away with things. I quite like knowing that you are about to do something bad and doing it anyway.''

Something bad? Such as what? Bad as in goosing-the-vicar bad? Or bad as in getting-whacked out-of-your-head-on-Class-A-drugs, stealing-the-nearest-fast-car-and-driving-it-off-a-cliff bad? ''Not hugely bad, but slightly wrong.'' That'll be more a ringing-somebody's-front-door-and-running-away kind of notion of badness, then.

Maybe I shouldn't be surprised. After all, it's the morning after the night before, when McAvoy attended a premiere of Inside I'm Dancing at the Edinburgh International Film Festival and, more dangerously, the after-show party. Yet he is impressively bright-eyed and probably bushy-tailed (there's only so much access you can get in these celebrity interviews you know and anyway, the lobby of the Sheraton is full of people having breakfast and we don't want to ruin it for them). All the more so since the PR was still trying to set up this morning's interview in the wee small hours of last night (her text message confirming the details of our meeting was timed at the back of 2am). If he is a real party animal, his powers of recovery must be pretty impressive.

In person McAvoy, 25, from Scotstoun, on the west side of Glasgow, is a slightly earnest young man who talks with some eagerness about the ''craft'' of acting (the director of Inside I'm Dancing, Damien O'Donnell, raves about McAvoy's professionalism). But he's likeable with it, chatting away about his love of cooking and camping (in the canvas-and-poles sense of the word), shouting out a greeting to Romola Garai, his passing co-star and apologising profusely when I have to buy him cigarettes because he has no paper money and his room account has already been settled.

He has, he admits, an excess of nervous energy. Between cigarettes he is constantly playing with the packet and the ashtray, or sweeping a hand through his floppy brown locks. It is all the more impressive then that throughout Inside I'm Dancing he is sat stock still in a wheelchair playing a character who is suffering from Duchenne muscular dystrophy, a life-limiting, muscle-wasting disease.

''For the first couple of weeks I was shitting bricks, just thinking, 'I can't act, I can't express myself, I can't flick my head coquettishly','' he admits, giving me an example of his coquettish head-flick - it's very good. ''What am I going to do? I can't do my tricks. And that is the thing. You're stuck in a scene with nowhere to go. I was terrified. But then, after a couple of weeks, it just hit me: 'F-, it actually gives me everything I need because that is a huge part of the character.' He is trapped every single day of his life, can feel himself physically degenerating.''

Rory's physicality turned out to be the key to the role for McAvoy. ''I had to hold my shoulder down, which isn't a hugely apparent thing in the movie but it helped make me look smaller. I couldn't get small enough to play someone with Duchenne muscular dystrophy. It would have made me anorexic to get that small but I got as small as I could. But having to hold myself in that particular way became incredibly tense. It gave me so much tension to my body it came out in my voice. So eventually it became something good, but there was two-and-a-half weeks of terror thinking I'm f-ed. People are going to find me out. This is the end of my career.''

Given his performance, that seems a pretty unlikely scenario. O'Donnell, for one, is something of a fan. ''The thing about James is he's a chameleon,'' he says. ''If you see him in Bright Young Things he's landed English gentry, if you see him in this film his north Dublin accent is just fantastic. I just think he's just a pure thoroughbred star.

''He couldn't even turn his head in this part and he still manages to hold the screen. I think he's on the way up. It's only a matter of time before he's doing huge movies.'' That may well be so but there are those who feel the part of Rory should have gone to someone who was disabled rather than just acting it. Hell, he's not even Irish. Nor is his co-star Steven Robertson. Disabled actor Jamie Beddard has said of the casting decisions: ''What does it say, when there is at last a high-profile film with two leading characters with disability and neither one of them is played by a disabled actor?''

McAvoy has heard similar sentiments. ''I've had disabled people saying that to me,'' he concedes. But he's not buying it. Indeed, he is actively bullish about taking the role. ''There's loads of disabled actors but I would love you to find an actor who has Duchenne muscular dystrophy who has had three years' training and any sort of experience, because their life expectancy is so short.''

Anyway, he says, he thinks the film may actually inspire people with disabilities to consider acting. It's a positive story, he says. ''This film isn't all about people getting their arses wiped by their f-ing carers. So maybe in future it won't need to be able-bodied actors (in such roles).''

Although no fan of method excess - ''I don't take my work home with me'' - it's clear McAvoy is serious about his profession. ''If you treat it like a craft and work hard and care about it then it's worthwhile because there is something noble about it,'' he says at one point. One could easily put this down to ''wanker actor nonsense'', if McAvoy hadn't already beaten me to it. His earnestness is always tempered by a sense of the absurd. Still, he obviously has a high opinion of his profession. ''I've been very, very fortunate to meet brilliant real people,'' he says.

This is actors we are talking about here, isn't it? It just so happens that a couple of days before I meet McAvoy I read an article by the actress and comedienne Arabella Weir in which she berates the latest in a long line of men she says she didn't sleep with. ''Guess what?'' she writes. ''It is another actor. 'Surely there can not be another vain, self-centred wanker who luxuriates in the belief that one glimpse of him transforms all women into quivering wrecks,' I hear you cry. Newsflash: apart from Jim Broadbent and David Tennant, they're all like that.''

So Mr McAvoy, defend your fellow actors. ''I'd say she is a very intelligent woman who's got great insight into the acting profession.''

That's not to say that he sees himself as one of those vain, self-centred onanistic types. His role models are the actors with integrity, the John Simms and Bill Nighys of this world. Not the wankers, then? ''Not the wankers, no. I can still manage to be a huge wanker at the same time as trying not to be a wanker.''

Maybe it is understandable that he should mount a defence of his breed. After all, acting has been good to the Scotstoun boy. Inside I'm Dancing is a success in Edinburgh, winning the audience award. To its credit, it stays just the right side of sentimental. But it's only one feather in McAvoy's cap.

Next week, he can be seen in a small part in the Paul Bettany/Kirsten Dunst romance Wimbledon and, once he's finished this interview, he's going back to London to pack his bags for New Zealand, where he's about to spend six months playing Mr Tumnus - a faun - in the big-budget adaptation of The Lion, The Witch and The Wardrobe alongside Tilda Swinton and Rupert Everett. ''It's a cliche but it was one of my favourite books when I was a child,'' he says. ''It terrified me but I loved it.''

Acting, it should be said, also gave him the opportunity to meet his current partner, Ann Marie Duff, his co-star in Paul Abbott's autobiographical Channel 4 drama Shameless (which returns in a new television Christmas special this year). They started as screen lovers and now are the real thing.

The last thing he did before coming to Edinburgh, he tells me, was make her a big breakfast (he'd probably just popped round). ''We had Tolouse sausage, scrambled egg ...'' Tolouse sausage? ''French sausage with garlic, pork. It's a kind of prosciutto itself. I love a bit of fancy sausage, but I like a bit of square sausage too.'' There speaks the grandson of a butcher.

It's not something he particularly wants to talk about but obviously things are going well with Ms Duff. I wonder if it's difficult for relationships that begin on set to be sustained in everyday life? ''No, I don't think so.'' There's no danger of getting your emotions mixed up with those of the characters you are playing? ''That can be a confusing thing for actors,'' McAvoy admits, ''and it can be a very exciting thing. It's very exciting to be on set. You're in an institution that creates itself in a week, then maintains itself for three or four months and then disperses. And it becomes a very close thing and it's very easy for passions to be magnified. And that's brilliant.'' But it's not him. ''I don't think Ann Marie and I would still be together if that was the case.''

If not quite a cocky little bugger then there's no doubt that McAvoy is not suffering from any lack of self-assurance. Given his background, some might be surprised by that. A few journalists in the past have tended to reduce his back story to a string of tabloid headlines of the ''boy from a broken home makes good'' variety. His parents split up when he was seven and McAvoy was brought up by his grandparents on his mother's side. In the past he has said his dad ''was probably not the best dad in the world'', someone who had married too young and became a father too young as well.

One could read all sorts of things into the fact he insists he has no memories before he was seven apart from the odd random flash. But this doesn't necessarily add up to the ''strange background'' some journalists have suggested. It's not as if his parents were the first couple to ever split up. And anyway, he says, his grandparents, James and Mary Johnstone, were more than adequate substitutes. ''They were always very supportive as long as I was doing something I wanted to do. They'd brought up five kids so (by the time he arrived) they'd worked it all out.''

James and Mary encouraged him when he got his first acting job at the age of 16, a part in David Hayman's Glasgow thriller The Near Room. He wangled the part after introducing himself to Hayman after the actor/director had given a talk at McAvoy's school. The film remains little seen. Just as well, McAvoy would tell you. He always says he was terrible in it.

He then worked in a bakery for a couple of years, thinking about what to do next. ''I thought I can either go to university and do a degree that I'll drop out of after a year or I can give acting a bash. So I auditioned for one drama school (the Royal Scottish Academy of Music and Drama) and luckily I got in. If I hadn't got in I wouldn't be doing it.''

Although for most of us he announced his arrival with last summer's BBC double header, State of Play on BBC1 and the less heralded but equally good sitcom Early Doors on BBC2, he's been working solidly since leaving drama school. ''I've never been out of a job,'' he says.

Still, it took him a while to find his feet in the profession. In the first few years, he kept threatening to give it up. ''I was very much like 'acting's a pile of shit. I should be doing something worthwhile, saving the world or something like that'.''

At which point, one of his fellow actors turned around and told him not to be an idiot (the words McAvoy uses are ''f-ing prick''). She told him he either should respect the job or give it up. He opted for the former option.

When he's not acting he spends time cooking and eating (it's hard to believe given the size of him - ''I eat a hell of a lot of food, man''), pitching tents in the Lake District or even bungee jumping. ''I like things like that, insane, slightly inappropriate things just for effect,'' he says. ''You're only testing yourself when you're slightly scared.''

So there we have it. James McAvoy is a feel-the-fear-but-do-it-anyway kind of guy. Which is pretty cocky when it comes down to it. Careerwise, he's still enjoying the thrill of the drop. Restraints haven't kicked in yet. The way he's going, it's hard to believe they ever will. n

Inside I'm Dancing goes on general release

on October 15. Wimbledon opens in cinemas this Friday.