The first half of The Big Sleep follows Chandler's novel almost exactly, including dialog. The major change in the first half is that Rusty (Sean in the movie) Regan (Rutledge in the movie) is never married to Vivian Sternwood. The second half differs from the novel by a fair amount and I don't know why really. It isn't better than the book. Whatever screenwriters (which in this case include William Faulkner) want to make their mark. Anyhow the movie is still good.