Most people will tell you to read the book before watching the movie...I really don't think that it is a good idea...either way! Reading the book first or watching the movie first won't make any difference! For me, it's much much better if either I watch the movie OR read the book. Not both.
You know why? Because obviously there are parts of the book that won't make it into the movie and for me every sentence in the book is important! Crucial! ESSENTIAL! I'm not blaming the director of course...I blame myself for putting too much pressure on thinking about how wonderful it would be if it were the same as in the book. I'm mildly disappointed...now I really wish that Stephenie Meyer hadn't written the book so brilliantly.I am not an imaginative person but reading twilight, breaking dawn, eclipse and new moon had really opened my eyes and mind that there are more to life than just logic.
Other alternative, watch the movie ONLY! The movie would probably disappoint you less if you don't know how the story goes according to the book. Even though you would definitely missed out on most of the so-called "unimportant" parts of the book.
Take my advice! You either watch the movie or read the book. Don't do both! You'll be disappointed! I would at least!
oh ok..that's all for now!