I didn't know he did an uncredited pass on Bonnie and Clyde (1967) until I read this interview with the director Arthur Penn:
Cineaste: Robert Towne received a credit as “Special Consultant.” What was that for?
Penn: He wrote certain little scenes in the film as well as some additional dialog, but very telling dialog. In the family reunion scene, for example, when they go back to visit Bonnie’s mother, that scene was in the original script, but it didn’t include Clyde’s explanation to Bonnie’s mother about how as soon as everything blew over he and Bonnie were going to settle down and live right down the road from her. And she says, “You do that and you won’t live long.” That’s Towne. He made some very salient contributions.I think that is why Robert Towne is so good.
He makes even the small scenes interesting.