Kenneth Lonergan plays Minister Ron in two major scenes counseling Terry at Sammy's house, and then counseling Sammy in his office. Since Lonergan had a lot of dialogue in those two scenes, he turned over the directing to Laura Linney and Mark Ruffalo, respectively.
Kenneth Lonergan, a first time Director, had protection from studio interference by his Executive Producer and friend, Martin Scorsese. They had met when Scorsese's then-girlfriend Illeana Douglas had auditioned for one of Lonergan's plays.
The scene with Rudy walking home from school in the pouring rain was created by the local fire department spraying a hose from their highest ladder.
This was Rory Culkin's first major movie role. Kenneth Lonergan was impressed on the whole with his acting ability, but had such a difficult time getting him to laugh, or even smile on camera, that he had Mark Ruffalo tickle him a few times.
Matthew Broderick, a friend of Kenneth Lonergan's, was the first actor cast in this movie. He visited Lonergan at his apartment, read the part of Brian, and asked if he could audition for it. Lonergan then told him the part was already his.