What are the differences between Gone Girl the movie and Gone Girl the novel?

One of the most important differences between the book and the film is the portrayal of Nick. In the film, he is much more of a conventional hero, someone for the audience to root for. Gone is the complexity of his character in the book, in which he is portrayed as being as much of a codependent as Amy. The relationship between Nick and Amy is much darker, more twisted, and much more interesting in the book. However, as Gone Girl the movie was always intended to be a mainstream Hollywood film, it is not surprising that the script irons out one or two of the original source material's complexities.
The character of Amy also undergoes some degree of change in the transformation from page to screen. In the film, she is presented as a typical femme fatale. This is in contrast to Nick, who is given to us largely as a hapless dupe. We do not get as much of a sense of the full range of Amy's psychological states as we do in the book. It is notable that the character of Hilary Handy is omitted from the film. She is one of Amy's high school classmates, and Amy falsely accuses Hilary of stalking her. Hilary's experiences serve to highlight Amy's insanity and how cold and calculating she can be in setting people up for crimes they have not committed.
There is a sense from the movie that Nick, in resuming his relationship with Amy, is largely going through the motions. As the hero of the movie, he is going to give their relationship another chance; he will try to live up to the ideals of the media-generated image of their marriage. In the book, Nick is strangely excited to be back with Amy and to resume their messed-up, codependent relationship. He knows who she is; he knows she is a stone-cold murdering psychopath. However, he is so addicted to Amy that he just cannot help himself. Although Nick is not a murderer, he is still severely messed-up psychologically. However, we really get no sense of that in the movie. Again, Nick is a fairly mainstream Hollywood hero, so he cannot be portrayed as too psychologically complex.
https://time.com/3466109/gone-girl-movie-book/