Despite a somewhat creepy screen presence, actor Paul Bettany really is a cheery fella. The rising star talks to Donald Clarke about playing his first romantic role in Wimbledon, midnight knocks on the door from Lars von Trier, and the good life as Jennifer Connelly's significant other.

In an age when British movie stars are expected to be either fey, stuttering imbeciles or glacially repressed enigmas, it is a pleasure to be able to celebrate the off-centre charms of Paul Bettany. Tall and thin, with sharp blue eyes, he's not a bad-looking fellow, but - on-screen at least - a slightly sinister energy hangs around him.

Bettany was perfectly cast in Paul McGuigan's Gangster No 1 as a villain who would grow up to be Malcolm McDowell. Like the star of A Clockwork Orange, Bettany has the look of a man who, even if he is being a total darling right now, intends to do something quite awful as soon as the camera chooses to point itself elsewhere.

So what exactly is he doing in Wimbledon? The latest romantic comedy from Working Title, perpetrators of Four Weddings and a Funeral and Notting Hill, stars Bettany as an ageing (that is to say, over 30) British tennis player who falls for Kristin Dunst's umpire-roasting American youngster just as he hits an unexpected run of form. It will come as no surprise to learn that the script, which has been gathering mould for some time, was originally written with one H. Grant in mind. The film's director, Richard Loncraine, was not immediately convinced of Bettany's appropriateness.

"He said to me, 'I really don't think you are right for this'," the actor explains. "Clearly I was on some sort of list of people that he was being forced to see, and it really made me laugh that he said it. I had never made a romantic comedy and I fell in love with him for being so frank."

After some consideration, Loncraine became convinced that it might be interesting to have a proper actor in the role after all and gave Paul's agent a bell. Good luck to him. Now what else can we say about the film without being impolite? Well, it looks as if Bettany has trained hard for the tennis sequences.

"It was a nightmare," he says, before catching hold of himself. "Oh God, I sound like an actor now. Well it was as much of a nightmare as it could be when you are being paid huge sums of money to play tennis."

But he's still on the fags, I see. "It's amazing the quite mesmerising number of cigarettes you can smoke when you are fit. You can smoke a hell of a lot without getting out of breath. That really is the only good thing about exercise."

A female colleague who interviewed Bettany earlier in the day was effusive about his twinkly, slightly flirtatious charm. With me he appears happy to slump into lad mode. One of the many excellent things about Paul Bettany is that, like his buddy Russell Crowe and unlike many American actors, he enjoys drinking, swearing and occasional bouts of shouting.

"Oh God, I love my Guinness and my Jameson's. I've just realised you're Irish and it sounds like I am just saying this. But it's true. I am naturally thin, but then you get down here." He pats a perfectly compact-looking belly. "And that's where I keep all the Guinness."

The 33-year-old Bettany, both of whose parents were actors, has admitted that in his 20s he did go off the rails somewhat. Would it be an exaggeration to suggest that, after attending drama school, he went through a period of what the tabloids like to call Drink 'n' Drugs Hell?

"No, it wouldn't," he says. "I have friends who do drugs recreationally, but it was always more of a full-time job when I did it. I wasn't very good at keeping it a weekend thing. And coincidentally - it must be a coincidence - the moment I stopped doing that I started to get regular work. I could never work on drugs. 'Oh God, everybody is looking at me! Oh no, they are looking at me, because I'm on a film set.'"

Bettany first attracted attention when he appeared in Stephen Daldry's acclaimed production of J.B. Priestley's An Inspector Calls for the Royal National Theatre. A number of small screen roles followed, but it was not until 2000 and his startling performance in Gangster No 1 that Hollywood began to take notice. A year later, in the entertaining A Knight's Tale, he was quite hilarious as an arrogant and frequently nude Geoffrey "Geoff" Chaucer.

"Nude scenes are really hard if you are stupid enough to do them sober," he says. "I have never been that stupid. So frankly a lot of Jameson's got me through that. And if a producer goes, 'That's really unprofessional,' then I say, 'Fuck you! You take your clothes off in front of 2,000 people.'"

Bettany's reputation grew. But, until Wimbledon, mainstream directors, perhaps concerned about his undeniable oddness, seem to have been reluctant to cast him in leading roles. He has twice played second fiddle to Russell Crowe - in A Beautiful Mind and Master and Commander - and found himself scurrying between Helena Bonham Carter and Olivia Williams in Thaddeus O'Sullivan's The Heart of Me. He did have a central role in mad Lars Von Trier's Dogville, but that was more of an ensemble piece. And, anyway, Von Trier is hardly a mainstream guy.

"One night there was a knock on the door of my hotel," Bettany says. "And this voice says, 'It's me, Lars.' Of course it was fucking him. Who else would it be at four o'clock in the morning? 'Paul. Can you lend me some underpants?' I open the door and there he is in the common passageway with no clothes on. So I gave him my underpants. I never got them back and I think I am quite grateful for that."

All sensible people like Paul Bettany, and you can see why. Aside from being genuinely nice (Loncraine tells me he is the most considerate actor he has come across since he directed Michael Palin in The Missionary), he is very funny and tells stories well without ever falling into lovey-darling, theatrical anecdote mode. It's little wonder that the actress Jennifer Connelly, his co-star in A Beautiful Mind, fell for him so quickly. I understand they didn't live together before they were married. How quaint.

"It just worked out that way," he laughs. "It had a really odd beginning. We met on a film set and didn't have a relationship, because they never work in those circumstances. And then we met up about a year later and I thought she was frankly gorgeous. Then later we went to Scotland, which it turns out is the Las Vegas of Europe: they will marry anybody there. We got married and she took me home like some Russian mail-order bride. So, yes, it was really Victorian, but we didn't plan it that way."

Since the birth of his first son, Stellan (named for his Dogville co-star, Stellan Skarsgård), Bettany, feeling that it's not desirable for both parents to be shooting at the same time, has cut down his work rate. "So when Jennifer was doing a film recently I went along and became the on-set bitch: a role I was surprisingly happy in." He will, nonetheless, find time to appear in a thriller with Harrison Ford next year. Life sounds rather comfortable.

"Yes. But I always feel that, at any moment, somebody is going to knock on our door and say, 'I'm really sorry, but we meant all this for somebody else. Can we have all this stuff back?'"

Wimbledon is released on September 24th