Gone Baby Gone” is one of writer Dennis Lehane’s Boston-set mysteries, this one surrounding the case of a missing child. Filming on a tight schedule, Affleck knew that going back to his hometown made everything easier. But after “Mystic River” and “The Departed,” the first-time director also knew that comparisons were inevitable.

“That was really scary, in one way, because how could you ever possibly measure up?” he says. “By the same token, I just convinced myself, OK, we won’t measure up to those movies. They will always be Clint Eastwood and Martin Scorsese. But we were coming from a different place — the smaller underdog movie.”

For Affleck, what makes “Gone Baby Gone” real is his dependence on real people, not actors. “If there weren’t enough people on the street, we would have to do some door knocking,” he recalls. “One of the advantages of being a known actor is that people weren’t outraged when you bang on their door. It was spectacular how people would just walk right out of their life and right into the movie.”

As a first-time director, Affleck took a lesson from one of his first helmers, Richard Linklater, who put him through the paces on “Dazed and Confused.” He even quotes directly from a letter that Linklater wrote to his young cast.

“I definitely encouraged the non-professional actors to contribute their own dialogue. Just the way people talk changes from year to year,” Affleck notes. “The Lehane book was written 10 years ago, the screenplay was written four years ago. So when somebody didn’t feel comfortable with the script, I said, ‘How would you say it?’ My entire directorial strategy was about creating an environment where I wanted the actors to be able to succeed as best as they possibly could.”