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.
Jack Nicholson once said of co-star Marlon Brando appearing in this movie, "The ground quaked for weeks before he arrived." Nicholson added that Brando was "exceedingly cooperative" and "gentle as a lamb."