Before shooting, Gene Hackman and Al Pacino both dressed as hobos and hitchhiked through California to get into their characters.
Gene Hackman's line to the waitress, "Is this your first day?", was an ad-lib. Hackman was genuinely annoyed with the actress.
According to Jerry Schatzberg, Gene Hackman was hard to work with and argued with everyone on the set including his brother, Richard Hackman, who was working as his stand-in. To get back at Gene, Schatzberg gave Richard a part in the film, but Gene ended up being delighted that his brother was in the film.