What is different
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The primary difference is the way in which the movie ended and the book ended. In the movie, it appears that the Indian character lifts up the sink, throws it out the window and escapes. In the book, it was somewhat different, as we are allowed to envision Chief's escape and a part of his journey.
Also,in the film, Cheswick supports McMurphy until the end, whereas in the book, after standing up for the first time against the Nurse, Cheswick and McMurphy have a short talk, and later that day Cheswick drowns in the swimming pool.
References Kesey, K. (1962). One Flew over the Cuckoo's Nest. New York, New York: The Penguin Group. Retrived June 24, 2015 http://www.onlinemoviewatchfree.com/watch-full-movie-one-flew-cuckoos-nest-online-now-online-movie-