Lupin Explains Sirius's Trick on Severus Snape

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Lupin reveals how Sirius Black once played a dangerous trick on Severus Snape, leading him to the Whomping Willow in hopes that he would encounter Lupin in his werewolf form. James Potter, hearing of the prank, saved Snape from certain death, though Snape glimpsed Lupin's transformation and held a grudge ever since. Lupin admits that Snape never trusted him and has opposed his appointment at Hogwarts.