Fresh off his directorial debut, Ben Affleck says he's found his calling. "In the beginning, part of wanting to be a director was just a natural extension of acting," said Affleck, whose movie "Gone Baby Gone," opens Friday. "But now this feels like what I am, or what I want to be. It's so satisfying and exhilarating.

"In fact, the central preoccupation of my life right now is trying to find another movie to direct," he told The New York Times for a story in Sunday's editions.

Affleck co-wrote the script and directed "Gone Baby Gone," a crime thriller set and filmed in Boston about the search for an abducted 4-year-old girl.

He told the newspaper he included as many locals as he could in the film, people plucked off the street or discovered in bars, even for speaking roles. One woman was cast as a beer-drinking smart-mouth after approaching him and saying, "I should be in your movie."

"I wanted something raw and authentic and even a little scuffed up," said Affleck, who grew up in the area. "People go to the movies to see something they can't get otherwise, and I thought this was a chance to take you somewhere that you couldn't otherwise get to the Boston you never see in the movies."

Say the name Ben Affleck, and myriad images come to mind.

The loyal Red Sox rooter and John Kerry campaigner. Half of the Oscar-winning Matt-and-Ben "Good Will Hunting" duo. Another half of the tabloid-fodder "Bennifer" couple. A sometimes-solid actor ("Hollywoodland," "Boiler Room") prone to weak movie choices ("Bounce," "Daredevil," "Gigli").

"Gone Baby Gone" will leave you with a new picture: filmmaker.

In his directing debut, Affleck has found his calling, an avenue for using his obvious intelligence while getting out of the way of his own celebrity. Co-writing the script with longtime friend Aaron Stockard, Affleck presents a place oozing with atmosphere and rich, complicated characters. He has enough confidence in himself (and in us) never to go for the safe, easy answer.

The film is based on the child-abduction novel by Dennis Lehane, author of "Mystic River," which earned several Oscars under Clint Eastwood's direction. Like "Mystic River," "Gone Baby Gone" is set on the rough streets south of Boston. Much has been written about Affleck's decision to populate the background of this gritty crime drama with authentic locals; he even gave several of them speaking parts during crucial scenes in run-down bars. It was a bold move that paid off big-time. You feel as if you've been immersed in an insular neighborhood, where your block is your entire world and secrecy is a critical trait.

As one of Boston's highest-profile native sons, Affleck clearly knows this area well. You won't find any regional cliches here — no obvious cutaways to foliage or Fenway Park. (Though he does thank Sox stars David Ortiz and Manny Ramirez in the closing credits.

Maybe they helped out as grips on the set one day.)

Working with two-time Oscar-winning cinematographer John Toll and composer Harry Gregson-Williams, Affleck never sugarcoats or overdramatizes the surroundings, instead establishing a mood that's fraught with danger, suspicion and dark discoveries around every corner. It's depressing and bleak, but also excitingly alive and real.

Private investigators Patrick Kenzie (Casey Affleck, Ben's younger brother) and Angie Genarro (Michelle Monaghan), partners in work and love, are hired by the family of the abducted 4-year-old girl Amanda McCready. The relatives figure that no one in the neighborhood wants to tell police if they saw or heard something, but they'll talk to a local guy they know and trust.

Patrick and Angie persuade the chief of the Crimes Against Children department (Morgan Freeman, among the superb supporting cast) to let them tag along with the detectives assigned to the case. Remy Bressant (a powerful, unpredictable Ed Harris) and his partner, Nick Poole (John Ashton), grudgingly agree to meet and talk with them, but Patrick suspects from the start that they're not being entirely forthcoming.

This is a hunch that will hold true for nearly every single character in "Gone Baby Gone," including Amanda's mother, Helene, a difficult role masterfully portrayed by Amy Ryan. Helene is an alcoholic, junkie, sometime-prostitute and drug mule. She probably loves her daughter but is so screwed up and selfish, she's also unapologetically neglectful. This is someone it would be easy to hate; Ryan makes her such a complete human being, you walk away feeling angry, yet with a glimmer of unexpected sympathy.

Just when you think Patrick and Angie have solved the case, another wrinkle emerges — then another and another. Affleck keeps you hanging on, wondering where he's going next, but never overhypes the proceedings. And the younger Affleck serves as an engaging figure to help us navigate the story's many turns. He comes off as a regular guy who can be both forceful and frightened, and his character ultimately faces decisions that would trouble anyone.

That pervasive sense of moral ambiguity is one of the strongest elements of "Gone Baby Gone," and it will you keep you thinking and talking about the film long after it's over. This much is indisputable, though: Ben Affleck is a director who surely has more great things in store.

"Gone Baby Gone," a Miramax Films release, is rated R for violence, drug content and pervasive language. Running time: 114 minutes. Four stars out of four.

"I know a few people who would play the part," the Jersey Girl and Gigli star continued. "Hell, even I would put some projects aside to do it if that's what the producers wanted."

According to Affleck, the fourth film in the Bourne franchise, which has traditionally taken place in European locales but was most recently set in New York City, needs a refreshing location that would surprise filmgoers, such as gritty South Boston. Just moments after the opening credits, the amnesiac CIA assassin would learn that he grew up there, had an abusive childhood, dropped out of high school, and once worked as a janitor at a local college.

Affleck said he ultimately imagined the film as being less a traditional spy thriller and more about friendship and the importance of realizing one's true potential.

"The sidekick represents the kind of life Bourne would have lived had he never gone off and become a hotshot CIA agent with a big-time career," said Affleck, who described the new character as having suffered a string of bad luck and made a number of poor life choices. "Basically, his character would kill to have Bourne's natural fighting and spying ability, but it's his role in the movie to show Bourne that even though he is a very gifted and talented individual, he should never leave his old friends behind."

Though Affleck, who offered to write the script for free, anticipates more lengthy scenes of dialogue than is typical for a Bourne film, he promised the movie would not be without its signature fight sequences. Affleck said he had already storyboarded one scene, set at a Boston Red Sox game, in which Bourne, the sidekick, and another high school friend, played by director Kevin Smith, get into a fast-paced drunken argument over the film Star Wars and each of their respective mothers.

"How do you like them apples?" said Affleck, referring to what the sidekick could theoretically say after Bourne breaks the necks of several security guards at Fenway Park.

Affleck, who won a Teen Choice Award for his work in 2001's Pearl Harbor, said he plans to pitch the idea to Damon the next time the two see each other, hopefully at an Emmy Awards after-party next month. If Affleck loses his invitation for the event Damon is attending, as he reportedly did last year, he and Damon will discuss the project during one of the rare weekends in which Damon is not working on a film and both actors happen to be at home in their native Massachusetts at the same time.

"I really think he'll go for this," Affleck said. "I know Matt Damon better than anyone in the world, and I just know he's going to love this. I know it. He just has to."

While admitting that a fourth Bourne film has not been discussed, executive producer Doug Liman agreed that a sidekick character might be worth exploring. Liman wrote in an e-mail Friday that he already had a short list of possible candidates for the role, including Vince Vaughn, John C. Reilly, Luke Wilson, Tim Robbins, Minnie Driver, Philip Seymour Hoffman, Casey Affleck, Don Cheadle, and Adam Sandler.