Continuity mistake: In the scene where McMurphy gets upset while playing blackjack, he gets up and walks towards the nurses' quarters to turn down the volume on the record player. Martini and Bibbit are still sitting at the card table while McMurphy is walking towards the nurses' quarters but when he gets inside, Martini is already leaning against the counter of the nurses' quarters with Bibbit close behind him. (00:27:00)

One Flew Over the Cuckoo's Nest (1975)
Plot summary
Directed by: Milos Forman
Starring: Jack Nicholson, Louise Fletcher, Michael Berryman, Peter Brocco, William Redfield
Genres: Drama
Randle Patrick McMurphy (Nicholson) has been dating a fifteen year old (fifteen going on thirty-five) and is sentenced for a short term for contributing to the delinquency of a minor. Rather than spend his time in jail, he convinces the guards that he's crazy enough to need psychiatric care and is sent to a hospital. He fits in frighteningly well, and his different point of view actually begins to cause some of the patients to progress. Nurse Ratched (Fletcher) becomes his personal cross to bear as his resistence to the hospital routine gets on her nerves.
McMurphy: What do you think you are, for Chrissake, crazy or somethin'? Well you're not! You're not! You're no crazier than the average asshole out walkin' around on the streets, and that's it.
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Answer: It's better explained in the novel, where the Chief is the narrator. Essentially, he played deaf and dumb, so he'd be left alone by the Nurse and the staff; seeing what they did to the other patients, he figured the less he was noticed, the better. In this way, he is privy to a lot of the seedier goings-on, since they don't think he can hear what they're saying or tell anyone what he's seen.