Hang-dog character actor Paul Giamatti never thought he'd make it big in films, but here he is, starring in the critically acclaimed sideways and brilliantly supporting Russell Crowe in Cinderella Man. Donald Clarke meets the unlikeliest - and most unpretentious - of movie stars

We are in New York and Russell Crowe is late for his press conference. When he eventually turns up he will explain that the delay was a result of his decision to walk to Central Park South from his downtown hotel. Then he will ramble on at such length that I am forced to leave early for fear of missing Christmas. Then he will fly to Manchester and back. Then he will hurl a phone at one Nestor Estrada. Oh, you know the rest.

While the Antipodean bruiser is ambling uptown, Paul Giamatti, his co-star in the boxing drama Cinderella Man, is dutifully working his way through his own interviews. It is hard not to draw comparisons. Crowe: the noisy star who was nowhere then suddenly everywhere. Giamatti: the hang-dog character actor whose ascent was slow and steady.

I really hope that Giamatti is not going to tell me that Russell was an absolute darling on set. Surely there must have been at least one outbreak of unacceptable behaviour.

"No. He never hit me with a frying pan, I'm afraid," he says. " I'm afraid he was lovely. He said to me: 'I can be a complete irascible asshole,' but he never was to me. I never saw him do the stuff you hear about him doing. He is an intense guy, a complex, moody guy. But not in bad ways. He is a madman in ways you might find unexpected."

He presses wild flowers? He folds origami butterflies? "He is a vulnerable, neurotic guy. 'They are always on my back about something I said in an interview,' he says. I have been with him at these interviews and he is absurdly open. 'If you don't want people to get stuck into you about these things, then just shut up,' I tell him."

Giamatti begins all his sentences with a hint of an exasperated wheeze. This, combined with his heavy, drooped features, has helped him establish a reputation as one of cinema's most effective malcontents. He played the superhumanly miserable cartoonist Harvey Pekar in American Splendor. He was the hopeless wine snob in Sideways. He was a little cheerier in Planet of the Apes but, with all that make-up on, who knew it was him?

Now 38, Giamatti began acting while a student at Yale University (of which his late father, a distinguished academic, was president), but never imagined that somebody with his looks could ever become a star.

"Oh definitely. My film career was not something I took very seriously. I had a stage career and that was my life. I mean, a guy like me couldn't have a film career, right? I'll play third cop, fourth cop from the right in films while playing good things on stage. The film thing was kind of a lark. I'll do this to make a bit of money. To my great surprise that became the career."

Hollywood requires those at the centre of its stories to come across like minor deities, but it also needs ordinary looking Joes and Janes to lurk in the background. Such character actors have tended to enjoy greater longevity than the Chiclet-toothed Scientologists whose drinks they serve and whose cars they chauffeur. Giamatti may not have secured his modicum of fame until the start of this decade, but he has been with us for quite some time. That was him being shouted at by Howard Stern in Private Parts. He was also in Saving Private Ryan and The Truman Show. A character actor is a good thing to be.

"I used to think so. Now I am not so sure." He laughs. "It's not that clear what that means anymore. It was used so specifically in the past. William Demarest, Wallace Beery: people like that. I'm not sure that category exists anymore. In my case, now I also get to play lead roles and that's welcome. But what is also good is that I now get much more interesting supporting roles. Previously I would just play the depressed writer guy or whatever. Now those roles have become much more interesting."

Nothing much of Giamatti shouts Hollywood. Articulate yet unpretentious, he has recently moved from Manhattan to Brooklyn with his wife and young child. He admits that if the acting hadn't worked out he would probably have followed his father into academia. Yet those unlikely lead roles did come his way, and he now finds himself a kind-of-sort-of celebrity. Last year, when Alexander Payne's Sideways emerged as the most positively reviewed film of the year, everybody expected an Oscar nomination to advance Giamatti one rung further up the starry ladder. It never came.

"Oh, sure, it would have been nice, but I certainly wasn't spitting with rage," he says, and, considering his pessimistic demeanour, one is inclined to believe him. "I will tell you what was a pain in the ass: all these people who assumed I would be upset. There were all these pats on the back from people and that drove me absolutely crazy."

He goes on to bemoan the publicity machine that sputters into action when it is perceived that a nomination is imminent. What will Giamatti wear? Who will he thank if he wins? All that is likely to start again next year when attention turns to his impressive supporting role in Cinderella Man.

"I know. I honestly don't relish all that. It is really intense for those months. But, of course, it would be really nice to be nominated."

Sadly, Ron Howard's Cinderella Man's many virtues have been overshadowed by chatter about perceived failures in its marketing. Telling the astonishing story of the comeback of Depression-era boxer Jim Braddock, the film received strong reviews but was knocked out in the first round by disappointing box-office returns (other, even less secure boxing analogies, litter the trade press). Some blamed the unhappy vibes generated by Crowe's fracas with Señor Estrada. Others felt the picture just looked too gloomy. But the general feeling was that the studio had screwed up by releasing a weighty film at the start of the summer silly season.

At any rate, despite the under-populated cinemas, the picture really works. Infinitely superior to Howard's previous collaboration with Crowe and writer Akiva Goldsman - the dishonest, cowardly A Beautiful Mind - Cinderella Man manages the tricky feat of being inspirational without being nauseating. Giamatti laughs in agreement. "Yes, I can do without inspirational films as well."

Giamatti plays Joe Gould, Braddock's gutsy trainer. Both men are financially devastated by the Depression but find a way back into the big time when Braddock, then barely scratching a living as a stevedore, is offered the chance to get the crap beaten out of him by an up-and-coming contender.

"It is a very different business to playing somebody like Harvey Pekar: he was around the set. And, anyway, I was playing the Harvey he had created in the comics, not the real guy. But the facts about Gould are known. I talked to his son and others who had known him. But we were also trying to hark back to those characters in boxing movies of the 1930s." Played by guys like the aforementioned Wallace Beery? "Yeah. Exactly like that."

One understands the desire to secure the film a less prohibitive certificate, but these are the least foul-mouthed boxing folk one could ever hope to meet.

"Oh yeah, and that did bother me a little," Giamatti laughs. "It's a very tricky thing to do. I may be the only guy that swears in the film. I say horseshit and bullshit. I said motherfucker once by accident and they had to shoot an alternate version: 'Yeah. That was fine Paul, but you said motherfucker.'"

Angelo Dundee, Mohammad Ali's legendary trainer, has a supporting role in the film. "Yes, and he's sitting next to me and he does nothing but swear. Cocksucker, motherfucker. Fantastic. But he was like this wonderful imaginative resource for me. I would ask him what's another way to say 'Hit him with your left' and he always had an answer. Wonderful."

Cinderella Man should help secure Giamatti's position in the business. His name is first on the list for directorsin search of charismatic glumness. Does he have any particular ambitions still to fulfil?

He shrugs. "I've never been particularly ambitious about all this. All I ever wanted to do was make a living doing good plays on Broadway. That was my only ambition and I have done that. I just want to keep working."

An amateur psychologist might suggest that Giamatti is deliberately suppressing expectations.

"That is definitely right," he wheezes. "I definitely have curtailed expectations in my life so that I won't be disappointed. Look, if this thing keeps going the way it's going that will be fine. It's already gone a fuck of a lot further than I thought it would."

Cinderella Man opens next Friday