The more I have experience of this, the more I've started thinking it should be 'movie first, then book'
Or if not possible: leave at least half a year between the book and the movie
I have much too much evidence of me not being able to enjoy a movie simply because it doesn't correspond to my idea of the book.
Harry Potter movies (yea gods... how are they even understandable without reading the books??)
Jack Reacher movies (Tom Cruise *is not* Jack Reacher!)
Valerian movie (very little to do with the comics)
Ready Player One (huge mistake to watch the movie the next day after finishing the book which I had liked. I was hopping mad & absolutely furious about the movie).
In most cases there's nothing really wrong with those movies AS MOVIES. Had I never read the stories, I'd probably been "yeah, ok, nice entertainment".
But in those cases it was *really* hard to get over the books. They had got under my skin somehow, so I had a big emotional reaction. Couldn't detach myself from the original story - couldn't view the movie as a separate work of art.
On the other hand: loved the LotR movies, loved the books. Have always liked most of A. Christie's filmatisations. Hugh Laurie and Stephen Fry's Bertie and Wooster series is a real joy. I'm a die-hard Jane Austen fan but I've never shrank from watching the movies -- period versions, modernizations, bollywood version - anything goes. With Austen, it's a bit different though, my attitude is close to "academic mindset" in this case; I'm just so curious to see what different kinds of things can be done with Austen's novels. Everything from Bride & Prejudice to Clueless to Pride and Prejudice and Zombies
Sometimes I know not to risk it: to my huge surprise, I totally fell for the Twilight books, and made an immediate decision to NOT watch the movies, as I knew they would never ever be able to make them the way I had envisioned. (I got a glimpse of one of the movie posters - yes, I also tried to avoid all pictures of the movies - and I knew I made the right decision... they looked Wrong!).
Even though I said above that maybe it should be movie first, then book, I'm not sure it would work for me... There have been several cases where finding out that a movie was based on a book has not in any way encouraged me to read the original work. Even if I had enjoyed the movie. I'm sure there are many examples but right now only two come to mind: the Divergent-series (I've seen the first 2 movies) and The Maze Runner series.