Sirius Black : Enough talk, Remus! C'mon, let's kill him!
Professor Lupin : Wait!
Sirius Black : I did my waiting! Twelve years of it! In Azkaban!
Professor Lupin : Very well, kill him. But wait one more minute. Harry has the right to know why.
Harry : I know why! You betrayed my parents! You're the reason they're dead!
Professor Lupin : No, Harry, it wasn't him! Somebody did betray your parents, but it was somebody who, until quite recently, I believed to be dead.
Harry : Who was it then?
Sirius Black : Peter Pettigrew! And he's in this room, right now! Come out, come out, Peter! Come out and play!
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From: Harry Potter and the Prisoner of Azkaban (2004)