What are some of the differences between the book & the film One Flew Over the Cuckoo's Nest?
Jack Nicholson does a fantastic job in the movie, but as an actor, his personality tends to dominate the characters he plays. In the book, by contrast, I think we get a more in depth, less quirky portrayal of Randle McMurphey, which to me really adds to the story.
I also think the movie focuses really heavily on the struggle between McMurphey and Nurse Ratched. While it is a central theme to the story too, the book does a better job of developing the other characters in my opinion.
The primary difference between the book and the film is that the book is written from a first person perspective, and is effectively narrated by the Chief. But since this same character is pretending to be mute, it would have been impractical and perhaps too experimental to also have him narrate the film via voiceover. The film has a more straightforward point of view, with no one character's view point predominate.
One of the things that is always different is the dialogue. It ends up being condensed in a movie. 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.