Paul Giamatti: Mr Potato face

He may not be your averagematinée idol, but plenty of people look just like Paul Giamatti – and someone has to play them, he tells Gill Pringle

With his receding hairline and nervous mannerisms, Paul Giamatti is not exactly your average leading man. And yet, in the three years since he wowed critics with his role as a melancholy, middle-aged wine aficionado in Sideways, he's swiftly risen to become one of Hollywood's most bankable actors.

At 40 years old, he's actually younger than Hollywood hunks like Brad Pitt, Tom Cruise, George Clooney or Johnny Depp, but his appeal has little to do with looks or youth; it's his very ordinariness that makes him a director's favourite. "Let's be honest about this. I'm limited by my looks so I'm a tough sell," he says, smiling apologetically. "But there's plenty of folk out there who look just like me, and someone has to play them. "

The youngest of three children, Paul Edward Valentine Giamatti was born into a high-achieving family: his father was Yale University's youngest ever president, later being appointed president of the National League of baseball, as well as writing six books. His mother, Toni Smith, was an actress before becoming a teacher and homemaker.

Following in his father's footsteps, Giamatti earned a master's degree in fine arts from Yale, also majoring in drama at Yale's School of Drama. But success in his career came slowly: he toiled in regional theatre for several years before Broadway beckoned in the mid-Nineties, when he performed in Tom Stoppard's Arcadia and David Hare's Racing Demon, later receiving rave notices for his roles in Chekhov's Three Sisters and Eugene O'Neill's The Iceman Cometh.

When he was offered the part of Miles in the low-budget Sideways in 2004, it wasn't quite the career move he'd anticipated. "It was a great script, but I didn't think anybody was going to care about a movie about two drunks in California," reflects the actor, who won a Golden Globe nod for his efforts. "I was shocked by the reaction to that film. I look awkward and pained a lot of the time in it, although I think that it was probably me who felt awkward and pained the whole time.

"I actually found it a very hard part to play because of the comedy of it. I've never really thought of myself as that funny, particularly. And on film it's a lot harder because you really can't tell if you're being funny or not. Comedy is harder because you can misjudge it really badly – it can be too much or too little.

"I'm not putting myself down, but I always want to do better than just a good job. I can't stand watching myself on screen. I've gotten more used to it but I'm still very critical, although I try not to be because it's actually not terribly helpful," he says.

Giamatti's father died 18 years ago while his mother died from cancer just as Sideways began to receive a buzz: "I got depressed at the time because it was like my best audience had died," he says. "My dad never saw me in anything. It's weird not to have any parents alive."

Family has had an impact on Giamatti's latest film, too. In Fred Claus, he plays Santa, whose wayward older brother Fred (Vince Vaughn) is seriously envious of his sibling. As Santa, Giamatti is clearly the favoured son; in real life, too, Giamatti's career has taken off in a way that his actor brother's hasn't. Marcus Giamatti, 46, launched his career before Paul, appearing alongside Paul Newman and Joanne Woodward in 1990's Mr & Mrs Bridge. Today, however, he works largely on stage and in television.

"I suppose everybody has, to some degree, a sibling rivalry thing," Giamatti says hesitantly. "I always feel like there wasn't really any with my brother, but there probably was more than I realised. He used to really beat on me, so clearly there was something going on. [But] he's actually had a really good career. He's done a lot more theatre and television than movies so he's always had a perfectly fine career. So we've been OK with that. I think it's just other people bringing it up that drives him nuts."

A self-professed atheist with a Jewish wife, Giamatti doesn't usually make a big deal out of Christmas: "Growing up, I don't think I ever believed the Santa Claus thing," he says. "It made no sense to me, it didn't add up, so I never bought it. For my own son, it's going to be really confusing because my wife is Jewish so we celebrate Hanukkah. We haven't done Christmas ever. My son knows what Santa Claus is, but I don't know what he's going to make of it this year. He's going to wonder why Santa doesn't come because [he's seen me in the suit and] I am Santa!"

Described unkindly on one website as "potato-faced", Giamatti says he doesn't mind not being a screen idol just so long as he keeps getting hired: "I've got to be the geekiest guy in the world in many ways, but it doesn't bother me. I've never considered myself to be an interesting person and I still have the mentality of a supporting actor. I don't see that changing."

Ask if he secretly longs to break out of type-casting and play a screen sex god, he almost jumps from his seat in shock: "A sex god?!" he splutters. "Well, the guy I played in Shoot 'Em Up [opposite Clive Owen] is kind of a sex god. I sort of molest a dead body and that was the sexiest thing I've ever done in a movie. But I've never really done the sex god thing, so I'd have to see what it would be like. Probably Santa would be easier for me."

He's just happy to be working, he says. "For years I wasn't able to pick and choose my roles, so that's a great thing for me now, although it's also daunting. Being able to vary things is great because I get bored with myself, as an actor. I like to challenge myself and find things that will be difficult for me to do.

"And when I'm not working, I go crazy trying to figure out what the hell I'm going to do next. I used to do yoga classes but now I get so paralysed because I want to work again and then I get sort of lost, so then I feel I can't waste doing my time doing yoga when I should be reading scripts."

'Fred Claus' opens on 30 November