In one scene where Herman Barker (Don Stroud) punches Ma Barker (Shelley Winters) in the nose, Stroud accidentally punched Winters in the nose for real, and hurt her badly enough that she had to be taken to the hospital.
Shelley Winters played a character that spoofed "Ma" Barker. On Batman (1966), she played "Ma" Parker in a couple of episodes as a guest star.
In a January 2003 interview with Esquire Magazine, Robert De Niro says he spent a lunch break sleeping in an open grave to help stay in character.
Dirkman (Bruce Dern) is a combination of several Barker Gang members, most notably Alvin Karpis, who was a cellmate of Freddie Barker and the primary member of the gang (outside of the family).