Paul Giamatti: Hollywood's humble everyman comes out of his shell

He isn't leading man material and he has a reputation for being reserved, but Paul Giamatti is in demand

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. "And I was thinking, 'People are reading this and thinking I'm a total fucking psychopath!'" 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. We're sitting on a sweltering day in an airless Venice hotel, and he's clearly suffering in the heat. "Is the back of my suit all sweaty?" he asks. A patch of moisture has soaked through the oak-green fabric, just underneath his shoulder blades. When I tell him it has, he shrugs. "Yeah, it's fine." Most actors would run for their nearest make-up artist if a bead of sweat appeared on their person. Likewise, most wouldn't turn up with bitten nails.

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 to date as a cocksure boxing promoter) 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' rival. The crux of the film comes when the ruthless Duffy lures Gosling's character for a meeting about swapping sides, a move designed to destabilise his opposition.

"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 naïve 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". Those yet to mature are Gosling and Evan Rachel Wood, alongside "the adults in the room" played by Giamatti, Hoffman and Clooney. "It's a grim view of being a grown-up. It's just a bad adult world that they inhabit."

The appeal for Giamatti was to play a character who, despite limited screen time, casts a "shadowy presence" across the film – often talked about or glimpsed on TV in the background (echoing Giamatti's own early career). It's not his first political piece: he won an Emmy for the 2008 mini-series John Adams, in which he played the title role, the second President of the United States. The way he sees it, the backstabbing that goes on now is nothing compared to back then. "The level of personal attack that went on is actually much worse than it is now. They were deeply, personally insulting to each other, in the way you can't be now. You have to be better behaved."

Raised in Connecticut, the youngest of three children, Giamatti comes from a family steeped in the liberal arts. His older brother Marcus acts, his Irish mother taught English and "was very briefly an actress", and his half-Italian father was a professor of comparative literature at Yale. Did Giamatti consider an academic career? "I thought about that. But it didn't happen. I had other interests – acting and animation, though that always seemed like a pretty tough racket." He still draws now, making his casting in American Splendor spot on.

Following his father to Yale, Giamatti graduated with an MA in drama, but never felt confident in his abilities. "It took a long time for me to feel like I could call myself an actor," he says.

His first "meaningful pay cheque" came at New York's experimental theatre La MaMa: "It was no more than $200. I did a weird play in which I played a leg-less, leper monk... it was an opera about a saint in Ireland! I had to drag myself around, and I hung myself at one point. It was completely crazy."

Things didn't get much better when he started getting screen work. Early credits included "Man in Sleeping Bag" in NYPD Blue. His first speaking role, in 1992's Past Midnight, saw him play a mentally disabled stable boy. A decade later, he was further humiliated when tween star Frankie Muniz dyed his ulcer-inducing Hollywood producer the colour of a blueberry in Big Fat Liar. In between, he won bit parts for Sydney Pollack (Sabrina), Mike Newell (Donnie Brasco) and Woody Allen (Mighty Aphrodite and Deconstructing Harry) before anyone knew who he was.

Giamatti gained critical plaudits for Sideways, though he wasn't at ease with it: "The hype actually bothered me," he sighs. "The hype got on my nerves. Everybody is shouting at you and that annoyed me." But he was prevented from becoming a household name when he was cruelly overlooked for an Oscar nomination. When he did pick up an Oscar nod in 2006, for Cinderella Man, Clooney beat him to the punch, winning for Syriana. He admits finally getting nominated "made me feel more relaxed and more confident in some ways. Which surprised me, because I didn't think it would but it did."

While the leading man roles are still few, he's in demand. He's recently completed The Congress, a live-action/2D animated piece from Waltz with Bashir's Ari Folman. He also features in David Cronenberg's forthcoming adaptation of Don DeLillo's Cosmopolis, playing a stalker to Robert Pattinson's billionaire asset manager. "It all takes place in one day, and a lot of it takes place in the car. He's in a limousine, stuck in traffic the whole day. It's a wonderfully strange – very strange – movie." Then there's Rock of Ages, a big-budget adaptation of the 1980s-set hit musical, co-starring Russell Brand and Tom Cruise.

He gets to sing Whitesnake's "Here I Go Again" and Journey's "Any Way You Want It" here. "They're so over the top emotionally that it's really great," he laughs. It's not the first time he's sung on screen – he starred in the karaoke tale Duets 11 years ago – but it is his first all-out musical. "It was really liberating and really fun. I was a little scared of it, then once I started doing it, I could see why people loved doing it." He's playing "a scuzzy music industry guy" – yet another chance for people to confuse him with his characters.

In reality, he's a shy family man who lives with his wife of 14 years, Elizabeth, and their 10-year-old son, Samuel. He collects books – "hard to find, rare" editions – and about the most dangerous thing he seems to do is put on a Led Zeppelin album. So it's not hard to see why this New York Times piece affected him – to the point where, if you bump into Giamatti on the street, you might see him raise a smile now. "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."

'The Ides of March' opens on 28 October