I also have a theory about what's necessary for a book adaptation to work on the big screen. When I see a movie that's based on a book, I don't expect to watch the book unfold before me; after all, they are very different media. But I have two things I look for, and if I don't see them, I'm perturbed... intensely perturbed.
In my ever-so-humble opinion, the film should retain the intention of the story and the integrity of the characters. The story itself may change, but the intent - the theme/arc/lesson - should remain. Some characters may be melded together, and a character may be a different gender than what was in the book, but that character should reflect the same virtues, strengths, weaknesses and flaws of the book character.
So this is my theory about what makes an adaptation work. It will be put to the test tonight. (Hence this being Pt. I. I'll comment on Breaking Dawn Pt. II tomorrow.)
What do you look for in a film adaptation? Are my two little expectations too little? Should there be higher expectations for adaptations? Or should there be no expectations?