More Snape! (not the cat)

I finally got to watch Harry Potter 3 this weekend, and I was underwhelmed. The sound seemed very fuzzy and we didn't get a lot of background on Moony, Padfoot, and Prongs. When, if ever, will the movies reveal Snape's motivations? How will they convincingly depict the later books if these are not made plain? More Alan Rickman, please.