Paul Giamatti - Hollywood's everyman comes out of his shell

Two years ago, a piece on Paul Giamatti appeared in the New York Times. Written by the author Austin Ratner, cousin to comic-book creator Harvey Pekar – whom Giamatti so perfectly embodied in his 2003 breakthrough film American Splendor – it was "an odd article", according to the actor. A lament for the writer's own late father, who like Pekar had contracted cancer, it was inspired by the fact that Ratner, who lives in the same New York neighbourhood, had seen Giamatti in a children's playground, with his young son, and had "felt my father's presence".

"It's all about how I looked like a complete psychopath and he didn't want to come up to me and talk because I looked like I'd hit him or bite him," says Giamatti. He's exaggerating: Ratner wrote that Giamatti "gave off fairly clear signals that he didn't wish to be disturbed. In fact, he seemed to scowl and mutter to himself, while pacing the sidewalks." Even so, it unnerved Giamatti. "I was like, 'Jesus Christ, maybe I actually do!'"

Giamatti has made a career from playing curmudgeons: Pekar, his wine-quaffing writer in Sideways, a Golden Globe-winning television producer in Barney's Version. Even his Hollywood outings – a baby-hunting villain in Shoot 'em Up, a foul-tempered gangster in The Hangover Part II – see him cast as the "frustrated asshole bad guy". Maybe all this misanthropy and misery has started to rub off on him. "I'm sure I'm an asshole in my private life, more so than I think I am," he muses. "I don't think I'm an asshole but I'm sure that people think I am. I try not to be!"

In fact, Giamatti comes across as self-effacing, good-humoured and quietly humble. With his black-rimmed glasses and retreating hairline, Giamatti is hardly leading-man material (when M Night Shyamalan cast him as such in his modern fairy-tale Lady in the Water, he was a janitor with a stutter). Now 44, he's more at home supporting the A-list. Jim Carrey (in The Truman Show and Man on the Moon), Tom Hanks (Saving Private Ryan) and Russell Crowe (Cinderella Man, for which Giamatti won his only Oscar nomination) benefited from having him along for the ride.

His latest attempt to make others look good is George Clooney's The Ides of March, a potent political drama set around the race to win the Democratic presidential nomination. While Clooney's Ohio governor Mike Morris has the support of Ryan Gosling's ambitious press secretary and Philip Seymour Hoffman's veteran campaign manager, Giamatti plays Tom Duffy, the man orchestrating the campaign for Morris's rival. The crux of the film comes when the ruthless Duffy lures Gosling's character for a meeting about swapping sides.

"Throughout all of this, people keep saying I'm playing the bad guy, and how does that feel, and I keep thinking, 'I don't know I'm playing the bad guy in this. Really, am I playing the bad guy?' I don't think I'm any worse than anybody else." If the film is a little naive in its cynical portrayal of Washington, the way Giamatti sees it, if you "take away all the political stuff, it's a really dark view of becoming an adult".

In private, he's a shy family man who lives with his wife of 14 years, Elizabeth, and their 10-year-old son, Samuel. So it's not hard to see why this New York Times piece affected him. "I think I have had an eye towards trying to be more friendly," he admits. "So people don't think I'm such a horrible person."

James Mottram

'The Ides of March' opens on 28 October