Cast the Expert: Dr. Spivey was played by Dean R. Brooks, the real-life superintendent of the Oregon State Hospital (who also served as a technical advisor on the film).
Creator Breakdown: Emotionally strained by a demanding shooting schedule that kept him 3000 miles from his 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, De Vito sought the advice of Dr. Brooks, who assured him that there was no reason to worry as long as De Vito could still identify the character as fictional.
Creative Differences: Jack Nicholson and director Miloš 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.
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.
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).
Mean Character, Nice Actor: Louise Fletcher was so disturbed by her own performance that she couldn't watch the film for years. She was so upset with the fact that the other actors could laugh and be happy while she had to be so cold and heartless that near the end of production she removed her dress and stood in only her panties to prove to the actors she was not "a cold-hearted monster".
Having played an orderly in the stage production, Michael Douglas originally wanted to play Billy Bibbit in the film.
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).