here’s what i see when i read the harry potter books.
young neville? ralph wiggum.
ron? ron from kimmmmmmm possible
dumbledore? patrick stewart
and snape? jafar from aladdin. certainly not the snape they had in the movies.
alan rickman died yesterday, and the HP world is awash in horror. while he may have been a great choice, is physical appearance in the movies did NOTHING for me. he needed facial hair. he needed some unkemptness. he needed….to be the sheriff from robin hood: price of thieves.
you say alan rickman? first and foremost, PoT. then harry from love actually. then hans gruber. dogma, galaxy quest. his role as snape was nothing that moved me (i’ll probably get a lot of hate for this). 
imagination is a pretty amazing thing while reading. that’s why i never really enjoy movies made from books i really enjoy, especially ones where i read the book before the movie was even a blip on a radar. it’s the reason i never watched “the time traveler’s wife” and refuse to see a john green book movie. (especially looking for alaska. that is… no.)
of course there are exceptions: shawshank was so amazing in movie form that i picked up the book. lord of the rings was a lot easier to read after seeing the movies. i read the girl with the dragon tattoo and was disturbed, but not as disturbed as seeing the movie (although i enjoyed the movie as well [the swedish version]). 
turns out there will be no adaptation that matches your brain’s view of a book while reading unless you are the one making the movie. and while rickman did a darn good job at snape, it wasn’t what i was expecting and not at all what i wanted. but i’ll remember him in other roles, fantastically acted roles. “loxley! i’ll tear your heart out with a spoon!”