Matt Damon sweats beneath his bulletproof vest. It is past midnight and he's part of a team of heavily armed detectives about to raid a Boston crack house. "What the hell? What am I doing here?" the actor asks himself as the cops give the signal to kick down the door. Just another day on the set for one of Hollywood's most successful young actors? Actually, no. The raid was all too real - a drug bust Damon took part in as preparation for his role as a bent cop in Martin Scorsese's $200m (£100m) thriller The Departed, co-starring Jack Nicholson, Leonardo DiCaprio and Mark Wahlberg.
"I was a lot closer to the action than I was comfortable with," says Damon, 35, talking in the relative safety of a New York hotel room. "I'm not so sure I would do something like that again." The actor is now the proud father of three-month-old Isabella and adopted eight-year-old Alexia, having wed bartender Luciana Barroso late last year. He relishes his new-found domesticity, and didn't enjoy putting it in jeopardy. "That said, I'm sure I was in no real danger. They went in with twice as many people than they would normally, so..." he says, not looking entirely convinced.
"I also did a couple other things [with the police]. I listened in on a wire and went on these things called 'buy walks', where they send somebody in to buy drugs and then they walk away. They slowly build a case and then they do a 'buy bust' when the guy goes up, makes the sale, and then they come running from all directions and then the state police come and go, 'You're under arrest!'
"Coming from Boston, I didn't need to learn the accent although, prior to this, all I knew of the state police was limited to the times I'd been pulled over on the pike for speeding! So to get in there and really see what these guys do was great. Once you get on a film set the clock is ticking; every minute costs a lot of money. But when you're researching you can go at your own pace and so I spent a lot of time with these guys; just soaking it in, and you just start to pick stuff up," says Damon, whose crack house raid is echoed in a similar scene in The Departed, using some of the actual Boston cops from the real-life bust.
"In all Marty [Scorsese]'s films there's an authenticity that you just can't fake and it's because he uses a lot of real people and because his actors have access to these real people to get as much understanding of the people they're playing. Ultimately, it's a giant magic trick; we're just trying to be believable and if you're taken out of the movie at all, then we haven't done our jobs. So all this leg work that goes in beforehand is just so that when we show up, hopefully the process is pretty smooth and the result is believable," he says.
But the drug bust wasn't the first time the actor has taken risks with his life, having shed 40lb (18kg) for his role as a Gulf War veteran in Courage Under Fire. After the film finished, he piled on the pounds even faster. "I got an anorexia-like digestive condition that took me a long time to recover from," he said at the time. "I'll never do that again. No career or dream is worth paying for with your health."
While The Departed is in true Scorsese tradition - a gritty urban drama filled with violence and bad language - Damon himself espouses non-violence, something he inherited from his mother Nancy Carlsson-Paige, a professor of early childhood education. "My mother is a very radical lady and gave me an excellent education. Growing up, I wasn't someone who fought a lot or anything like that. I saw a lot of violent things happen, but probably not any more than most kids growing up in the city. I have an interesting time with this stuff because my mother specialises in non-violent conflict resolutions, so I hear from her about portrayal of violence in cinema all the time. The violence in The Departed, none of it is gratuitous and the characters all pay a price. And that's a good message to send out to people," he says.
Damon's parents divorced when he was just two years old, and he was raised by his mother and stepfather in a commune. At 22, he arrived in Hollywood to try his luck. With work thin on the ground - his first role was a one-liner in the Julia Roberts movie Mystic Pizza - he spent much of his time playing video games, eating junk food and brainstorming screenplay ideas with childhood friend Ben Affleck. "I was so low on the pecking order that if Brad Pitt, Johnny Depp and Leonardo DiCaprio and every other guy passed on a script, maybe I'd get an audition. Writing the screenplay was my way of saying, screw the system. I'm doing my own thing. It was like, 'Why are we sitting here? Let's make our own movie'."
As a result, Damon and Affleck spent the next three years hawking Good Will Hunting around Hollywood, refusing offers from those studios who simply wished to buy the screenplay in order to cast Pitt or DiCaprio in the two lead roles they'd reserved for themselves. Miramax would eventually take the bait, agreeing to cast the pair in the lead roles and paying them $1m for the privilege. That was 10 years ago and it's Damon who now calls the shots after winning an Oscar with the screenplay. Proving that Good Will Hunting wasn't just a flash in the pan, he immediately made three well-received films: Saving Private Ryan, The Talented Mr Ripley, and The Rainmaker. "Matt has got the gift - and he's a writer in his own right. That gives him something special," Francis Ford Coppola told Time, while Anthony Minghella observed: "There's something so apple pie about him. You know he was the best-looking kid in his school, won all the awards at track and field and dated the most popular girl."
If it was all "Matt and Ben" in the wake of Good Will Hunting, then today it is just as likely to be "Matt and Brad" or "Matt and George". Currently filming the final instalments of both the Ocean's Eleven and Bourne Identity trilogies, Damon has established himself as a box-office heavyweight. Not that he chooses his projects by the size of the pay cheque. In accepting his role in The Departed, he skipped the customary salary negotiations. "When I heard it was Martin Scorsese and it was set in Boston and Leo was aboard, I said yes on the spot," he admits. "It's really rare in a film of this budget to have characters that are this interesting. Generally, the bigger the budget, the less interesting the characters become, but all of us has real things to play and that's a credit to Bill Monahan and the script. To be able to have that much to do when you go to work every day was really great."
Next on Damon's to-do list is directing: "I can't wait. I have a couple of things that I'm looking at but it will probably be something ultimately that I write. Just do it small. Ben just did it this year and he loved it. It's like having kids, you know. In five years, you'll wanna do it, man!
"I've been working with all these guys [directors], hen-pecking them with questions, trying to get my bearings and see if I can take a shot at it. I wrote the Good Will Hunting script when I was 22 - and it's still the most fulfilling thing because we had control of it for so long. It existed because we'd made it up and that feeling is really deep and nothing else has quite compared to that yet. So I can imagine that directing is like that. I want to try it and hopefully I'm good at it because I have a feeling that will be a deeper deal."
'The Departed' opens on Friday