I can't concentrate if I'm reading a book after i've seen the movie
Let my imagination picture the characters, unless, of course, it's important for the story.
For example, no movie about vampires I've seen could describe the beauty, horror, attraction etc. for Dracula's brides better than Stoker made me see with my own mind! That's why maybe I'm dissapointed each time I see a movie. Not to mention that in a movie they leave out all the interesting stuff.
But ok, I guess there is an exception
: in a dirty book you need a lot of those physical details.