Jack Nicholson's acting persona as the heroic rebel McMurphy, who lives free or dies (through an act of mercy killing), had earlier been set with his performances in Easy Rider (1969) and Five Easy Pieces (1970). The mid-70s baby-boomers' counter-culture was ripe for a film dramatizing rebellion and insubordination against oppressive bureaucracy and an insistence upon rights, self-expression, and freedom.
The role of the sexually-repressed, domineering Nurse Ratched was turned down by five actresses - Anne Bancroft, Colleen Dewhurst, Geraldine Page, Ellen Burstyn, and Angela Lansbury - until Louise Fletcher accepted casting (in her debut film) only a week before filming began. And actor James Caan was also originally offered the lead role of McMurphy, and Marlon Brando and Gene Hackman were considered as well. The entire film was shot in sequence, except for the fishing scene (shot last).
It surprised everyone by becoming enormously profitable - the seventh-highest-grossing film ever (at its time), bringing in almost $300 million worldwide. The independently-produced film also swept the Oscars: it was the first film to take all the major awards (Best Picture, Best Director, Best Screenplay, Best Actor, and Best Actress) since Frank Capra's It Happened One Night (1934).