Bookslut, which kicks butt, on the three rules of adapting books into film. Yes, I know this is an old post but the insight is still good:

In using the rules to think about adaptations, the first thing to determine is what's important about the process, and what specifically leads to bastardizations of good books. The Scarlet Letter is a crappy adaptation because it takes a story about morality and hypocrisy and transforms it into a reflection upon Gary Oldman's naked ass (forever a symbol of forbidden passion). Fight Club is a great adaptation because it remains true to the novel's depiction of undermined masculinity. It may be impossible to adapt a story note by note. But holding true to the intention of the book will lead to a faithful adaptation.

Thus, the First Rule of Adaptations: A film adaptation may not, through omission or direct action, undermine or reverse the meanings and morals of the source material.