Matt Damon: 'I have turned down scripts if the violence is gratuitous. I do believe it has an effect on people's behaviour'

Matt Damon is almost visibly shivering. He's turned his mind back to the beginning of the decade, to a time when his career was on the skids. Robert Redford's golfing saga The Legend of Bagger Vance and the Cormac McCarthy adaptation All The Pretty Horses, both of which cast Damon in the lead, had tanked. Holed up in Paris, he was on the fourth round of re-shoots for The Bourne Identity. "All the indicators were that that was going to be a turkey too," he says. Going through his mind was the simple rule of baseball: three strikes and you're out. "Nobody had offered me a job in about nine months."

Then, sitting alone in his hotel room, he took a phone call. It was Steven Soderbergh, who – in Hollywood terms – was the hottest director on the planet back then, after his films Traffic and Erin Brockovich had both been nominated for Best Picture Oscars that year. He wanted Damon for a script he'd just received, based on Kurt Eichenwald's The Informant: A True Story. Even though the pair had just worked on the then-unreleased Ocean's Eleven, Damon's self-esteem was so low, he thought the director wanted him to adapt the book, not star in the resulting film. "It was the most unlikely phone call," he adds, "or it felt like it at the time."

Since then, of course, Damon has not looked back. Both The Bourne Identity and Ocean's Eleven were mega-grossing hits, each spawning two equally successful sequels. If that proved he was a commercial draw, he's used it as leverage to work with some of the most renowned talents in Hollywood, everyone from Terry Gilliam (The Brothers Grimm) to Martin Scorsese (The Departed) and Robert De Niro (The Good Shepherd). By any standards, it's been a remarkable decade. Damon simply shrugs. "If I could just keep working, with the people I've worked with already," he says, "I'd be happy."

It's 11am. Sitting in an armchair in the bar of Venice's luxurious Cipriani Hotel, Damon appears chipper considering it's the morning after. He went to bed in the wee small hours, following the world premiere of The Informant! at the Venice Film Festival. Seeing as it's taken Damon and Soderbergh seven years to get the film made – at one point, it was put on hold for 12 months so that Damon could make The Good Shepherd – you could imagine there was a fair bit of celebrating to be done. Refusing to hide his eyes behind his Ray-Bans, which he keeps folded in his hands, his blue eyes are fresh, his skin the same, and his jock-like physique toned and tanned.

Dressed in a black polo shirt, grey canvas trousers and a pair of scuffed black boots, Damon still looks as boyish as he did when he shot to fame in Good Will Hunting back in 1997. Written with his childhood friend Ben Affleck, while they were both struggling actors getting parts in redundant films like School Ties, the pop-psychology story of a school janitor/undiscovered mathematical genius won them both an Oscar for Best Original Screenplay – and Damon was afforded the only Best Actor Oscar nomination of his career to date, for playing the title role. All of a sudden, these two white-collar boys from Massachusetts became "Matt & Ben", like some sort of inseparable double-act. An off-Broadway play was even named after them.

Since then, Damon and Affleck have reunited on screen only briefly – as two earthbound angels in Kevin Smith's Dogma and in the same director's sickly Jersey Girl (in which Damon took a cameo as a PR exec). In that time, Affleck saw his career nosedive with a series of flops and endured a publicity-drenched engagement to, and split from, Jennifer Lopez. Damon, meanwhile, despite high-profile romances with Claire Danes, Winona Ryder and Good Will Hunting co-star Minnie Driver, kept his head down. For the most part, he also chose wisely, playing the title role in both Spielberg's Saving Private Ryan and Anthony Minghella's The Talented Mr Ripley.

It's evident he is still fond of Affleck, when I ask if they have remained good friends. "No! I threw him out!" he grins. "He's such a boring guy." He then tells me that Affleck is directing his second movie, The Town, back in Boston, where Damon was born. "It's going to be really good," he enthuses. He doesn't see Affleck as much as he'd like, he says. Partly because, for the past four years, Damon has been living in Miami with Luciana Barroso, a former barmaid he met in 2003 while shooting the Farrelly brothers' comedy Stuck on You and married two years later. Living far from Hollywood has its advantages, though, like escaping from the prying eyes of the paparazzi. "We haven't had any problems," he says.

If this suggests a sensible approach to dealing with his celebrity, so does marrying a woman who has nothing to do with the industry. Damon has always been down-to-earth, certainly compared to Affleck. Still, last summer, the old friends holidayed together in Hawaii. It made an interesting spectacle for celebrity watchers, as these two boyish talents were now, finally, family men. Affleck, along with actress-wife Jennifer Garner, has two daughters. Damon and Barroso have three – 11-year-old Alexia (Barroso's daughter from a previous relationship), three-year-old Isabella and 14-month-old Gia Zavala.

With all this in mind, it's little wonder that Damon is at ease with the prospect of turning 40 next year. "I feel good," he says. "I've worked really hard the past 12 years and I've really enjoyed it. I don't regret any of the movies that I've done or any of the choices that I made. Even the ones that didn't work, I got a lot out of them. I'm excited. I've got terrific kids and a great wife. I really couldn't have asked for anything more." So are you happy to get old? "I'm OK with it – unless you can tell me how to stop it! But the roles do get a lot more interesting as you get older."

In the case of The Informant!, it actually helped that Damon had aged seven years when he finally came to play the part. Set in the early 1990s, he plays Mark Whitacre, who became the highest-ranking executive in US history to blow the whistle in a case of corporate fraud. Whitacre winds up becoming a mole for the FBI when it becomes clear that his employers, Archer Daniels Midland, are involved in a global price-fixing scandal regarding the animal-feed additive lysine. Oh, and in case you hadn't gathered, this is a comedy. And Whitacre, who may just be the most unreliable narrator in film history, is a compulsive liar.

After playing second fiddle to the likes of George Clooney and Brad Pitt as the eager-to-please crook Linus Caldwell in Ocean's Eleven and its two sequels, Damon finally has a chance to take the limelight in a Soderbergh film – and he does so with aplomb. Early reviews have already singled him out for praise (US trade paper Variety noted he was "in very sharp form"), while he has evidently impressed his peers with his work. As Soderbergh later tells me, he screened the film to Clooney, who is credited as executive producer on The Informant!. "Clooney said, 'This is one of Matt's shining moments,' and he's right," says the director. "He's unbelievably funny in the movie. It's a great comic performance."

Wearing an assortment of props – a bulbous nosepiece, a wig and a ratty moustache – Damon also decided to pile on 30 pounds to play the role. In the past, he's gone to extremes in the name of his art, notably developing an eating disorder as he crash-dieted for his Gulf War-veteran junkie in Courage Under Fire. As the late Anthony Minghella said, "Damon believes good acting is only possible through pain". Scoffing pizza and drinking dark beer, however, was slightly less painful. "I just basically ate everything I could see for a few months," says Damon. "I loved parading around shaking my belly for my kids and stuff. They thought it was funny, the fact that I got all squishy."

The conversation inevitably turns to his character's compulsive lying. "It's so hard to lie because you have to keep track. But he's able to sit and have a conversation – like, 'There's a sale on ties at the store' and 'I buy my ties in Paris' – because he's operating on all these levels." What about Damon himself? He looks far too wholesome a chap to tell fibs, right? "It takes a lot of energy to lie, which is why I don't do it, because I'm not any good at it," he says, with a tone that reeks of honesty. "If you're not accustomed to lying, you're invariably going to trip yourself up. Which makes it fun to play a guy like that because he's never wrong-footed."

Certainly Whitacre seems to fit the mould of characters to which Damon is frequently drawn. Think of his memory-addled spy Jason Bourne, a man who's desperately trying to discover who he was before the CIA trained him up. Or his Tom Ripley, who assumes the personality of the millionaire playboy he kills. Even Linus from the Ocean's films spends much of his time in disguise, while his career-criminal in The Departed is instructed to infiltrate the Boston police force and remain in deep cover. Identity crises looms large in his work, something that has lent Damon – rather unfairly – a reputation of being something of a blank slate.

In person, he's nothing of the sort – and certainly not like the verbally challenged version of himself seen in the Trey Parker-directed puppet satire Team America: World Police, where all he can do is shout out his own name. With politics that "skew to the left", Damon was perhaps ripe (and definitely game) for being mocked, but in reality, the actor keeps his philanthropic work to himself. After Good Will Hunting, he went back to Massachusetts to support a janitors' strike. He visited Zambia with the lobbying group Data, to publicise the importance of clean drinking water. And, in 2008, he co-produced Running the Sahara, a documentary about endurance athletes which supported the H20 Africa charitable foundation.

Much of this stems from his relationship with his mother, Nancy Carlsson-Paige, an author and "hippie" college professor who lectures in child psychology. She divorced Damon's stockbroker father, Kent, when the actor was two, and decided to raise him and his older brother, Kyle, alone. Damon's childhood sounds unconventional, to say the least. He and his sibling were encouraged by their mother to "be totally carried away by how we played". When young Matt was ticked off at school for staring out of the window, Carlsson-Paige told the teacher off. She then proceeded to complete her son's schoolwork herself while he played outside, under the belief that he was daydreaming in class because he already knew how to do it all.

With his mother specialising in teaching "non-violent conflict resolution", Damon admits that it has affected his choice of scripts. "Now I always look at the violence [in a script]. I don't want it to be gratuitous. Because I do believe that that has an effect on people's behaviour. I really do believe that. And I have turned down movies because of that." When Damon first became famous, his mother came out in the press stating that her son had become a product. "He's not a human being any more," she cried. "He's a cog in the capitalist system." Damon admits she still "doesn't think much" of the film industry. "She doesn't give a shit about the star system."

Yet she must secretly be proud that her son has rarely frittered his opportunities on disposable Hollywood fare. Damon's next film is Invictus, directed by Clint Eastwood and based on the book Playing the Enemy: Nelson Mandela and the Game that Made a Nation written by John Carlin, a former journalist on The Independent. Set in 1995, when Mandela was in his first term as South Africa's president, it follows his attempts to unite a racially and economically divided country at the moment it was due to host the Rugby World Cup. With Morgan Freeman cast as Mandela, Damon plays Francois Pienaar, the captain of the Springboks who steered his team to that year's final.

If Damon is enthused about working with Eastwood, it doesn't come close to how he talks about Mandela, whom he met on the shoot. "That was amazing. We got to take our kids. Lucy and I sat and just watched them. The stills photographer from the movie was allowed to come, and I was like, 'Take some pictures!' And so he took all of these shots of Gia – she was about six months at the time. And the kids were captivated by him. They just got it instantly. Looking in his eyes, they understood everything."

He trails off for a second, as he recalls the moment with a mix of pride and wonder. I tell him it will be remarkable for his children to see those pictures, when they are old enough to understand who Mandela is. "Some day, yeah," he says. "And Alexia, our 11-year-old, already does. She came to Cape Town, and some of her classmates came and they went to Robben Island, and they knew all about Mandela. So Alexia was very nervous. Isabella and Gia weren't at all because they didn't know, until they got into the room and were completely taken by him."

Alongside Invictus, he will also be seen in another politically charged film, Green Zone, based on Rajiv Chandrasekaran's 2006 bestseller Imperial Life in the Emerald City. Reuniting him with Paul Greengrass, the director of the two Bourne sequels, it sees Damon play a WMD inspector in the aftermath of the Iraq invasion.

It doesn't stop there, either. Damon is currently shooting The Adjustment Bureau, the story of an affair between a politician and a ballerina, alongside Emily Blunt. After that, another reunion awaits in January, when he will hook up again with Eastwood for Hereafter, a supernatural drama from Peter Morgan, the British screenwriter behind The Queen and Frost/Nixon. Then comes another film with Soderbergh, a biopic of Liberace, in which Damon will play the lover to Michael Douglas's take on the legendary entertainer.

While all of these reunions suggest that Damon is probably the most loyal actor in Hollywood, he knows the time has come for him to shed his skin and step behind the camera. "I can't wait to direct," he says. "I choose my movies based on the director and so I've been treating the last 12 years like a film school. And all the directors I've worked with have been very tolerant of my questions. The next step is for me to try it myself." He doesn't, however, have anything in development. "I'm not in a rush. The jobs that I'm getting not only are really exciting for me as an actor but I'm still working with people that I have a great deal to learn from." It's an attitude that does him credit – and explains why we've only just begun to see the best of Matt Damon.

'The Informant!' screens at the London Film Festival on 23, 24 and 26 October and goes on general release on 20 November