Well, you're not the only one, I suppose...You should look at my face when they told me that David Thewlis was supposed to play Lupin's character.
Also, I didn't find Gary Oldman as suitable for Sirius as I thought he should be. I couldn't have in mind the guy playing in the "5th element" for Sirius...! But then I realized that this actor is charismatic. He can play a variety of roles and he can change his appearance every time he changes a role.
That happened in PoA, too. What was I expecting? Find a Sirius looking stupidly in the screen having his head half shaved?
But than didn't happen with David Thewlis. I still can't cope with the fact that he is gonna be Remus Lupin!
Anyway, my point is that the people who just watch the movies (and don't read the books) don't have a right and copleted opinion about the characters.
I, personally, haven't harmed my imagination yet after watching the hp movies. But still I'm comgratulating myself for watching them after reading the books. That could ruin everything that I have in mind now.
Soooory for the long post!