Dr. Spivey was played by Dr. Dean R. Brooks, the real-life superintendent of the Oregon State Hospital (who also served as a technical advisor on the film).
Mel Lambert (the harbor master) was a local businessman, rather than an actor. He had a strong relationship with Native Americans throughout the area, and it was he who suggested Will Sampson for the role of Chief Bromden.
Jack Nicholson and director Milo Forman had a falling-out over McMurphy's motivation during pre-production of the film adaptation, leading to the two men speaking through the cinematographer and Nicholson not contributing anything to the film's DVD special features.
Mews Small, the actress who played Candy, claimed in a Q&A that Nicholson and Forman remained on speaking terms, and if they ever stopped speaking it was only temporarily while she wasn't around.
Ken Kesey grew more and more dissatisfied with the changes Forman was making to his original story and cut ties to the production near the end.
Creator Breakdown: Emotionally strained by a demanding shooting schedule that kept him 3,000 miles from his girlfriend and future wife, Rhea Perlman, Danny DeVito developed the coping mechanism of an Imaginary Friend with whom he would have nightly chats. Concerned that his own sanity might be slipping away, DeVito sought the advice of Dr. Brooks, who assured him that there was no reason to worry as long as DeVito could still identify the character as fictional.
Development Hell: After starring as McMurphy in a 1963 Broadway adaptation of the novel, Kirk Douglas obtained the movie rights and spent a decade trying to obtain funding. By the time his son, Michael Douglas, was able to secure a production deal, Kirk had gotten too old (and/or unbankable) for the role.
Disowned Adaptation: The only time Ken Kesey ever watched the movie was when he was channel surfing one night and randomly came across it. He changed the channel once he realized what it was. He even sued the movie's producers because it wasn't shown from Chief Bromden's perspective (as the novel is).
The script called for McMurphy to leap on a guard and kiss him when first arriving at the hospital. During filming, Milo Forman decided that the guard's reaction wasn't strong enough, and told Jack Nicholson to jump on the other guard instead. This surprised the actor playing the second guard greatly, and in some versions, he can be seen punching Nicholson.
Forman would roll the cameras when the cast members didn't know it, so he could capture the "real moment".
Method Acting: Many of the cast members stayed in character, even when the cameras weren't rolling.
One-Take Wonder: The final scene was shot in one take, whereas the party scene took four nights.
Scully Box: When McMurphy first meets the Chief, the Chief must be standing on a lifting box. You can't see his feet in the shot. Jack Nicholson is 5' 10". Will Sampson was 6' 7". In the shot, Nicholson's shoulders appear only to come up to Sampson's elbows. Also, the finished height of most doorways is 6' 8" and Sampson's head appears to extend higher than that in the shot. Even accounting for perspective, he might be on a 5" plus box. Nowhere else in the movie is the height difference so apparent.
The Indian war dance wasn't in the script. It was Jack Nicholson's idea.
According to Josip Elic, Nicholson improvised the scene where he gets on Brancini's shoulders to play basketball. Nicholson told him, "If I fall, I close this picture down for a week." Elic replied, "If I fall, I close this picture down for two weeks."
Having played an orderly in the stage production, Michael Douglas originally wanted to play Billy Bibbit in the film. Randy Quaid was considered for the aforementioned role.
Dan Blocker (yes, the same guy from Bonanza) was planning to produce and play the main character, but ended up losing the opportunity (this was shortly before he died of surgical complications in 1972).