Since I saw the movie before reading the books, main characters are like those in the movie. However, I imagined Sirus to be much diffrent (more handsome) and somehow Lupin in my head was wolf-like and also more handsome.
For Moody, I feel like I hear his wooden leg when he's walking and I've envisioned his eye to move all around his head (funny), so I'll probably be disappointed when GoF movie comes out.
Voldemort, on the other hand, is more like faceless for me. In GoF, I just can't envision him.