Sirius and Lupin Explain How They Used Their Animagus Forms to Control Lupin

Remus Lupin tells Harry and Hermione that Sirius Black and James Potter became large Animagi, which allowed them to control him when he transformed into a werewolf. With Peter Pettigrew's ability to slip past the Whomping Willow's branches, they were able to spend their nights roaming the grounds of Hogwarts. Lupin also reveals that this was how they created the Marauder's Map.