Stephen Mangan turns up at the Charlotte Street Hotel with rosy cheeks. He is tall, lean and athletic, flushed from cycling round central London. He goes most places by bike, sometimes cycling from his home in north London's Primrose Hill into town several times a day. He sits on the edge of a sofa in a back room, his rainproof jacket flung over a leather Mulberry satchel. He has that serious streak common to most funny guys but he also laughs a lot. I realise later, when listening to the interview, that he sounds a bit like a relaxed Tony Blair.

Mangan, 35, is such a convincing actor that I almost expect him to be as Machiavellian as the characters he has been playing of late. Yet there is no sign of the brash, narcissistic comedian he played in last year's Festival, Annie Griffin's excellent film portrayal of the Edinburgh festival. Not a hint even of Dr Guy Secretan, the insolent, sex-mad anaesthetist he portrays so brilliantly in the television series Green Wing. He doesn't seem to be as ruthlessly combative as his character in Confetti either; this new British film will cement Mangan's reputation as a great comedy actor but it may do nothing to enhance his reputation as a nice guy.

Victoria Pile, the creator, writer and director of Green Wing, is worried the hospital-based comedy may have ruined Mangan's career. She is joking, of course, but she has a point. "Steve's terrified because he's painted as such a hideous, ideologically unsound character in both series of Green Wing. He's so not like Dr Guy; of all the actors, he's probably the most opposite of his screen character. He should be proud of the fact that he's managed to be a complete bastard but retain some appeal. That's talent. Steve's always been a genius as far as I'm concerned."

Until Green Wing, Mangan was best known for playing the lead in Adrian Mole: The Cappuccino Years in 2001. Now he has to consider turning down what he refers to as the "arrogant bastard" roles. "It's a tricky one; it's the way it works in this business. If you play a part with any degree of success, you will probably be presented with other similar roles."

As soon as Green Wing was broadcast in late 2004, the offers to play "arrogant bastards" started - first in Festival and then Confetti. Festival gave him the chance to appear alongside the likes of Daniela Nardini and Chris O'Dowd while Confetti, starring Martin Freeman, Jessica Stevenson, Robert Webb and Jimmy Carr, is a homage to the current crop of British comedians. Both were so strong and funny that Mangan could turn neither opportunity down.

Typecasting may pose a danger but Mangan admits he enjoys playing insensitive, competitive guys. "The joy of being an actor is that you get to play characters who may be the antithesis of you. I love playing characters who have a misguided sense of how the world perceives them. People can say unspeakable things in comedy, especially in this country where we have a set of rules about how you're supposed to behave and what you're supposed to say. There's a code that stops people saying what they are really thinking." He laughs and there's a mischievous glint in his eyes. "It's fun to break that code. It can be liberating."

Confetti presented Mangan with the possibility of taking this notion a stage further; unusually for a feature film, it had no script and was improvised from start to finish. Director Debbie Isitt had the most basic of story lines - three couples compete to win the title of Confetti magazine's "Most Original Wedding of the Year" to win a dream home - but otherwise pretty much gave the actors a carte blanche to create their own worlds. The result is an enticing mix of Richard Curtis and Mike Leigh, a series of love stories that stop just short of sentimentality by showing how hard it is to make a relationship work.

Martin Freeman, whose pedigree after co-starring as Tim in The Office is impeccable, was initially reluctant to improvise, but Mangan relished the opportunity to be creative. "Some actors like improvising and others don't. It's nothing to do with talent. There are some fantastic actors who couldn't improvise a line. It's one of those things . . . " He searches for the right words. "If you enjoy doing things by the seat of your pants, not knowing what's going to come out of the other actors' mouths, it's fantastic."

Mangan and his on-screen fiancée (played by Meredith MacNeill) are professional tennis players with no friends. Mangan's character is increasingly jealous of their coach, which climaxes in a fight. "Jesus de Miguel is a real tennis coach; he came to teach us and Debbie [Isitt] offered him the part. He is tanned, athletic, very good-looking, charismatic. We got on very well off camera, but because he's Spanish, I'd run rings round him as we were improvising. One day he told Debbie I was annoying him so much he felt like hitting me. Debbie simply said go for it. I didn't see it coming at all. Nobody told me. I went for it, hit him back, but then I ran away."

Half-way across the tennis court, Mangan changed his mind and ran back. "I got about 10 feet away from him, running at full pelt and didn't have a clue what to do." He laughs. "I launched myself at him, slipped to the ground and he fell on top of me. I hurt myself quite badly. It's an extreme example of what can happen with improvising." It makes for good cinema; you can see very real fear on Mangan's face when he turns back to face his enemy again.

Improvising also appealed to Mangan because he spent the first five years of his career on stage, where he felt he had more control. "I refused to do anything but theatre when I first left Rada. Theatre is the actor's medium, film is the director's medium and on television the writer is king." That he attended Rada at all is lucky for those of us that can't imagine Green Wing without him. He studied law at Cambridge and acted throughout school and university but without considering it as a career. "It wasn't an option as far as I could tell. I'd have put it in a category with astronaut or jockey."

Mangan's parents were both born in County Mayo and although they moved to England in the 1960s, the young Stephen spent a lot of time in Ireland. "I was an English public school boy in Ireland so I always had a sense of how certain people can be perceived by other groups. Which is maybe why I tend to play those parts: I can understand Guy's brashness and arrogance. For him it's the only way for dealing with a very threatening world."

Just after Mangan graduated from Cambridge his mother died and he felt he had nothing to lose by applying to Rada. He runs his hand through his curly hair, looks at the floor. "Mum used to love the theatre; she took me regularly after I turned eight. She didn't know I became an actor . . . " Your dad must have been proud though, I suggest gently. He looks even more bereft. "He died last year actually," he says quietly. "But he was pleased, yeah. He often came down on set."

Sometimes Mangan wonders why he works so hard. After his father died, he realised he had spent most of his life trying to impress his parents - but now he was alone the incentive had faded and there was no one to prove anything to but himself. At other times he is very conscious of his mortality and feels there is no time to be wasted. He has spent the past 15 years thinking about writing and, in the spirit of getting on with things, is finally getting round to it.

"I've let myself off the hook by telling myself how busy I've been acting. So I'm going to clear a bit of space now and try to write." He looks worried. Does he have enough discipline? "I don't know. We'll have to find out. I love working with people, so it might prove tough, especially as I live alone." He pauses and, for the first time, sounds a little like Dr Guy. "But I do have a girlfriend."

Mangan says he's hard on himself. He worries about wasting opportunities, making the wrong choices. He feels these should be "great years", but as soon as he says it pulls a face; he doesn't want to sound pretentious in print. Talk of writing has made him edgy and he keeps running his hand through his hair. "I absolutely love acting, it's just . . . I don't know if it will always be enough. It's not often very intellectually challenging. I'm not professing to have a great brain but you do need to exercise it . . . "

He sits right on the edge of the sofa. "What if I try to write something and I'm rubbish? I've had some dream of writing for so long and . . . well, I'll probably be rubbish to start with." There's no way of finding out other than having a go, I say. It's not like you have to show anyone. He smiles, sighs and sits back. "I have to find out, you're right. My biggest fear is getting to 60 and realising I haven't had the courage to do something I always wanted to do."

"Confetti" is out now.

"Green Wing" is on Fridays at 9pm on Channel 4.