When McMurphy tells the Big Nurse that he'll think of her every time he swabs out a urinal.
[The Big Nurse] walked along [the toilet bowls] shaking her head and saying, "Why, this is an outrage...an outrage..." at every bowl. McMurphy sidled right along beside her, winking down his nose and saying in answer, "No; that's a toilet bowl...a toilet bowl."
When McMurphy takes the patients on a fishing trip, and pretends they're all doctors charting a boat.
McMurphy: This is Dr. Cheswick, Dr. Taber, Dr. Frederickson, Dr. Scanlon, the famous Dr. Scanlon, Mr. Harding, Dr. Bibbit, Dr. Martini, and Dr. Sefelt...Oh, I'm Dr. McMurphy, R. P. McMurphy.
The basketball game between the patients and the orderlies.
"Play the game, Harding! Play the game!" and other various ways Taber trolls Harding and Harding taking it all so personally.
The entire scene with the first group therapy where these two are really at it. Harding's freak-out speech takes the cake. Some of the scene was adlibbed and you can just tell how amusing Jack Nicholson found it watching the other actors.
Billy tries to chicken back out of a date with Candy when she breaks out with McMurphy in the middle of the night, insisting he'll do it when he's "got a free weekend".
McMurphy: What, ya busy right now? You got somethin' to do?
Turkle hiding along with the guys from the night superviser.
Randle tries to hook Candy up with Billy in front of everyone and Billy Nope's right out of the room. Randle sends every one of the guys to go chase him down and they bring him back out in a wheelchair.
Martini is in the wheelchair when they go after him.