Many extras were authentic mental patients.
Will Sampson (Chief Bromden) was a Park Ranger in Oregon near where the movie was filmed. He was selected for the part because he was the only Native American the casting department could find who matched the character's incredible size.
In later interviews, Louise Fletcher said that she found ways to make her character human, yet remain unsympathetic, ultimately deciding that Nurse Ratched actually did care about the patients, and felt she was doing what was best for them, but was ultimately misguided and drunk on her own power.
Jack Nicholson took a percentage of the profits in lieu of a small salary for a modestly budgeted film. The move paid off when the picture went on to gross well over $120 million. In his Oscar acceptance speech, he thanked Mary Pickford (who had received an honorary award earlier in the ceremony) for being the first actor in Hollywood to take a percentage of the profits.
During filming, a crew member running cables left a second story window open at the Oregon State Mental Hospital and an actual patient climbed through the bars and fell to the ground, injuring himself. The next day The Statesman Journal in Salem, Oregon reported the incident with the headline on the front page "One flew OUT of the cuckoo's nest".
Anjelica Huston: Jack Nicholson's one-time girlfriend appears as one of the crowd on the pier as the fishing excursion returns.