Now that I've had more than a couple of days to wait for New Moon, I've come up with a list of things I think the movie has to have to stay true to the book. In no particular order:
1. Both books share some similar plot points; it's all part of the story. But the moviemakers added some things into Twilight that are more like what happens in New Moon. The police tracking various attacks is one of these -- events are similar, but they need to be presented in a way that makes the story completely different. Because Bella is different.
2. Which leads us to our next must. New Moon has the ultimate sorrow. Bella really loses it, she drowns completely. That part has to be just as strong in the movie. Where you simultaneously just want to stop because it hurts so bad, but also have to keep reading/watching because you want it to end.
3. Along those same lines, the audience can't be given hope. You're left pretty hopeless in the book, even while you're intuition begs to differ. The departure of the Cullens (Edward in particular) has to be permanent; we can't be told that they're coming back into the story.
4. We have to see a big change in Jacob. First emotionally from the innocent, smiling character to the angrier Jacob, and also physically.
5. The Volturi have to be creepy. Volterra is my favorite part of this book; the Volturi are just plain weird. They can't always be understood, which is why they invoke fear. Keep them creepier than creepy.
6. There has to be a big difference between Bella and Edward's relationship and Bella and Jacob's. At this point, one is love and one is temporary comfort (where Bella is half-insane, so she really doesn't know much of what she's doing, anyway.) Movies love love-triangles; there's not really a love-triangle in New Moon. Almost, but not quite.
7. Edward's proposal should be as similar to the book as possible. I know they always have to change some of the wording and this and that, but it needs to keep as much of the same essence as can be done. It's perfect the way it is, plot-wise and emotion-wise; changing it too much would affect other aspects of the movie(s).
These are just minor hopes; they wouldn't be as big of deals:
1. The wolves are huge, not just big, huge.
2. When Alice comes back, there's a distance between her and Bella. Something edgy that wasn't there before. It dissolves quickly, but it creates the distrust you need as you slip into the truth of why the Cullens left.
3. Resist the urge for action. New Moon only has a little, and movies always like to up the action level. That's fine as long as they don't overdo it, just as long as you still look at it through Bella's passive eyes.
I'll probably come up with something else after I've seen the movie; I made this list pretty quickly. There's my experiment though. We'll see how it matches up to the movie in about twenty-five hours.