I'm a movie slut fan, so I always enjoy watching the actors portray the charaters, but I'd read the books before the movies came to the screen. I definitely had different images of them in my head than the actors, and those are the ones that have stuck when I do art.
I love drawing/painting adult Harry, post Hogwarts. He's my favorite. Filled out, slightly shorter than Snape; I really like picturing him in his 20's.