Matt, meet Marty

NEW YORK -- When Matt Damon first met Martin Scorsese eight years ago, he hardly expected he'd be starring in one of the director's films less than a decade later. The year was 1998 and the ``Good Will Hunting" star and co-scribe was mere days away from picking up the best screenplay Oscar -- and its subsequent fame and acclaim -- that would change his life. But for the moment, the Cambridge native was just an actor looking up to a cinematic god.

``I didn't really talk to him, just told him I was an admirer. It was kind of surreal," says Damon, showing no trace of having city-hopped four times in the past week. ``Ben [Affleck] and I were at a big party that our agency threw -- it was one of those deals where each of the three big agencies were throwing Oscar parties -- and Marty came to this one event. It was a huge deal for us."

Damon received ample opportunity not only to converse with, but also to actively collaborate with Scorsese on the set of their new cat-and-mouse thriller, ``The Departed," which opens Friday. A loose adaptation of the 2002 Hong Kong hit ``Infernal Affairs," the film features an all-star cast, including Scorsese muse Leonardo DiCaprio and an obscenely off-kilter Jack Nicholson, not to mention a tense nail-biter of a plot. Damon and DiCaprio play two rival undercover agents -- specifically, an Irish mob informant who has risen through the ranks of the Boston police force, and a clean cop who infiltrates the same gang -- each of whom is frantically trying to unmask the other before being discovered himself.

As Colin Sullivan, the dirty cop secretly reporting to Nicholson's brutal and increasingly unhinged mob boss Frank Costello, Damon deftly swings between displays of cold ambition and raw desperation. His Sullivan is a narcissist obsessed with the golden dome of Beacon Hill and the upscale life it represents, and is willing to get it whatever the cost. It's a multifaceted part that falls squarely into the 35-year-old actor's increasingly steady pattern of disappearing into intense and morally complex roles for talented directors.

``Matt's incredibly interested in who people are, and all he's concerned about is finding the truth of the moment and who his character is," says co star Vera Farmiga, who plays a police psychologist who gets involved with Damon's and DiCaprio's characters. ``Matt is so generous as an actor and emotionally direct, he was a real light on set."

Interestingly, neither Damon nor DiCaprio -- who also rose to superstardom in late 1997, for a small film called ``Titanic" -- initially knew which character he wanted to play. Their director, however, had no such doubt; Scorsese has said he immediately pegged Damon as the ambitious Colin because of his ``cocky attitude, a bravado."

Damon expressed no hesitation when first approached about the project.

``Brad Pitt's company [Plan B] had the script and he told me about it," says Damon, who's currently shooting ``Ocean's 13" with Pitt in Los Angeles. ``It was like one of those dream conversation s where someone comes up to you and says, `Martin Scorsese's making a movie in your hometown and he wants you to play the lead, are you interested?' I think I said yes before I even met with him."

The decision was no doubt influenced by the filmmaker's conspicuous absence from Damon's already packed resume of A-list directors. For those counting , the tally stands at three pictures with Steven Soderbergh (``Ocean's 11," ``12," and ``13"), three with Gus Van Sant (``Good Will Hunting," ``Gerry," and a small role in ``Finding Forrester"), and one each with Steven Spielberg (``Saving Private Ryan"), Francis Ford Coppola (``The Rainmaker"), Anthony Minghella (``The Talented Mr. Ripley"), Terry Gilliam (``The Brothers Grimm") , and Robert Redford (``The Legend of Bagger Vance").

``I want to direct someday too, so [working with Marty] was like yet another chance at film school," says Damon, who henpecked Scorsese whenever he found a quiet moment. ``All of my directors have been very forthcoming about why they're doing what they're doing, and everyone's approach is totally different. Marty's is based on having seen absolutely everything out there, so he knows every single way of telling a story.

``Plus, with Marty, if you ask him why the camera's moving, you're going to get a 10-minute explanation because he's got this encyclopedic knowledge of cinema. He's famous for that," he says. ``And the most interesting thing about this environment -- the thing most unlike any other set I'd ever seen -- was how quiet it was. The first day I walked on set, I stopped because I thought they must be rolling, but they were just setting up."

Damon isn't done with law enforcement just yet. In addition to ``The Departed," the Oscar winner is headlining December's ``The Good Shepherd," an espionage thriller about the haphazard early days of the CIA and one agent's struggle to preserve his soul over the course of a 40-year career. Executive-produced by Coppola, the saga marks Robert De Niro's long-coming sophomore outing behind the camera (after 1993's ``A Bronx Tale") and co stars Angelina Jolie -- better known these days as ``Departed" producer Pitt's well-traveled other half -- as Damon's patient wife.

Damon will then flout the law in the third installments of both the ``Ocean's 11" and Jason Bourne franchises. The currently shooting ``Ocean's 13," which is due out next summer, reunites the actor with Clooney & Co. plus two new playmates -- Al Pacino and Ellen Barkin. And Paul Greengrass's ``The Bourne Ultimatum," drawn from Robert Ludlum's novel, begins production this fall and is set for a 2007 release.

The actor's slate is also being filled by a documentary he's narrating and producing through his Live Planet production company. Titled ``Running the Sahara," the film -- which Damon announced during a fund - raiser for the nonprofit organization One x One at the Toronto Film Festival -- will follow three ultra-marathoners as they attempt to run across the Sahara Desert. But this isn't your average documentary.

``[The organization and I] realized it was a good platform with which we could launch this clean-water initiative," says Damon, who is amping up his charity work and traveled to Africa earlier this year. ``As these runners go across, we'll identif y sites where we can put in water programs and we'll go in with groups that are already on the ground."

Damon, while no stranger to activism, credits his status as a new father for his magnified look at the world's ills.

``Your relationship to all children changes once you have a child," says Damon, who married long time girlfriend Luciana Barroso in a quiet City Hall ceremony in New York last December, and watched her give birth to baby Isabella in June (he has also adopted Alexia, Barroso's daughter from a previous marriage). ``When talking about the world's children, it's easy to get overwhelmed by numbers and statistics, but once you have children, every single one becomes this beautiful soul. The older I get, the more I realize how much we can change."