I will make no secret of loving books and their movie counterpart, even if the movie turns out to be quite cheesy and not well-received. It’s always best when they get the casting so right it’s impossible to picture anyone else in the role. There’s been a lot of great casting but my pick is:
To Harry and his friends, Snape was a villain. No matter the reader’s view on the character(villain, misunderstood, double or triple spy, etc), in the trio’s eyes, he was someone to be feared and avoided and immediately suspicious of the second something went wrong(and it couldn’t be linked to Malfoy). Everything about Alan Rickman as Snape worked for me and I can’t even begin to picture anyone else playing the role.