While Lupin, Pettigrew, and Sirius' animal transformations are shown in action, they leave out the important backstory behind why these friends decided to become Animagi together. In fact, the film doesn't even reveal that James Potter is an Animagus, let alone an Animagus of a stag. The film never even makes the connection that the Marauders-Moony, Wormtail, Padfoot, and Prongs-are Lupin, Pettigrew, Sirius, and James, respectively. In the movie, by contrast, few of these details are provided, and most of them are simply glossed over in a stammering shouting match between Lupin, Sirius, and Harry. In the book, Sirius and Lupin lay out the truth contemplatively, explaining their friendship with James Potter and how Pettigrew framed Sirius for his betrayal. The film does reveal this, but it does so rather chaotically. Notably, one of the most important revelations in Harry Potter and the Prisoner of Azkaban is that Sirius Black is an innocent man, and the real enemy who ratted out Harry's parents is Peter Pettigrew ( Timothy Spall).