No! Never ever read books first. Always watch a movie first. Idk why I like going into movies with as minimal info about the storyline as possible. The plot's more surprising that way. Then, read the book later if you love the movie
It sucked whenever I read 'Gone With the Wind,' because the whole movie felt like it was in fast forward with how much they were rushing everything (bare in mind this is a four hour movie). The second half is so hard to watch now. It's the best part of the book too.
The reason why I think you should always read the book first is because it's simply the first and original idea and since the movie is in most cases worse you should be able to get better first impression.
I mean would people even get, let alone like the last 3-4 HP movies if it wasn't for the books?
