When Ben Affleck received the green light for his directorial debut, the first actor he turned to was John Ashton. You may remember him best as Detective Sergeant Taggart of the Beverly Hills Cop series. But to Affleck, Ashton was always Marvin Dorfler from 1988's Midnight Run, one of his all-time favorite movies. Casting Ashton, who plays a cop partnered with Ed Harris' Det. Remy Broussard, was just one of many personal decisions Affleck made as the helmer of Gone Baby Gone. The $19 million film is based on a novel by Dennis Lehane (Mystic River) that centers on two private investigators (Casey Affleck and Michelle Monaghan) who search Boston's seedy underbelly for a missing 4-year-old girl. When writing the screenplay, Affleck chose to make his protagonist younger than in Lehane's novel, a decision that gave his younger brother a plum role and let him avoid taking the part himself. ''Frankly, I didn't think I could do it,'' says Affleck. ''It took everything I had to direct this movie, and there are still a lot of things in there that I think, If I could go back, I might shoot that scene differently.'' So how was it for Casey, being directed in his native Boston by his big brother? ''Everywhere we went, I knew what the neighborhood was, who was living in those houses,'' he says. ''And living in Boston for 18 years made it easier to avoid the common pitfalls of changing your accent.''