Trivia: When Lupin is turning into a werewolf, Sirius says to him, "Did you take your potion tonight?" He is referring to a wolfsbane potion, which is not included in the film. According to the book, if a werewolf drinks the potion on the day before and the day of a full moon, he can keep his mind when he transforms. Lupin forgot to take it because he saw Harry with Sirius in the Shrieking Shack (on the Marauder's Map). Therefore, he became a fully-fledged monster.