The film, set in the early 1960s, is based on Donn Pearce's 1965 novel of the same name. Pearce sold the story to Warner Bros. , who then hired him to write the script. Due to Pearce's lack of film experience, the studio added Frank Pierson to rework the screenplay. Newman's biographer Marie Edelman Borden states that the "tough, honest" script drew together threads from earlier movies, especially Hombre, Newman's earlier film of 1967. The film has been cited by Roger Ebert as an anti-establishment film which was shot during the time of emerging popular opposition to the Vietnam War. Newman's character, Lucas Jackson, is described (by the notorious "Captain," upon his arrival at the prison), as a "free spirit," whose personal record (read out loud because of its unusual details) indicates a man who started well in the US Army—receiving medals for bravery in Korea—rose to the rank of Sergeant, yet was discharged as a "buck" Private.