I'd probably have a notepad and pen with me during the movie and writing down everything that happened and then ranting on and on about the differences after and talk about it like a conference with my friends and they all be like "What is her problem?"
See, this could be true and false. Take for example twilight. That movie series was a fail from the start when they casted plank as Bella. But then you have Harry Potter, The Lord of the Rings. Those movies are amazing, and they are very similar to the books. So, I guess it just depends on the book and movie...?