Marlon Brando's performance in this film was mostly improvised. Director Arthur Penn eventually gave up on him, and decided to just let him act whatever way he wanted.
Jack Nicholson did not like the fact that Marlon Brando used cue cards while filming. In their scenes together, Nicholson broke his concentration every time Brando shifted his gaze to the cue card behind the cameraman.
The title refers to the film's main setting, the rugged north-central Montana region, where the Missouri River is said to have "breaks", cutting into the land, due to the rough rising of the river.
According to Robert Silva's "Dustin Hoffman in Spurs? Method Actors Show the Old West Who's Boss", the whole production was affected by Marlon Brando's bizarre behavior, which allegedly included biting a frog and catching grasshoppers at the end of the day's shooting.
Original publicity reported that Marlon Brando ad-libbed so much, that a special stenographer had to be flown in from California to record Brando's ad hoc poetry and spontaneous dialogue.