In the thirdHarry Pottermovie -The Prisoner of Azkaban, Professor Severus Snape filled in for Remus Lupin as a Defense Against the Dark Arts teacher. It was a day before Harry’s Quidditch match and he and Oliver Wood were a bundle of nerves. And then, on top of everything else, the rooms and corridors of Hogwarts Castle were filled with talk about Sirius Black’s escape. He’d slashed the enchanted Fat Lady portrait that guarded the entrance to Gryffindor Tower and as the Lady herself confirmed, was somewhere inside the castle. Naturally, it wasn’t a particularly thrilling time for Harry as one of his favorite teachers was absent, and he still didn’t know why Arthur Weasley had warned him not to go looking for a deranged murderer.

In chapter 9 of the thirdHarry Potterbook -The Prisoner of Azkaban"Grim Defeat," Harry eavesdropped on interesting bits of conversation betweenSeverus Snape and Albus Dumbledorethat brought to light the former’s lack of faith in the Headmaster’s decision to appoint a teacher. “You remember the conversation we had, Headmaster, just before…ah…the start of term?” asked Snape, adding “It seems… almost impossible…that Black could have entered the school without inside help. I did express my concerns when, you appointed…” While Snape stopped short of naming this appointee, he was referring to Hogwarts' newest Defense Against the Dark Arts teacher, and Hogwarts alum, Professor Remus Lupin. Dumbledore shot down the theory that someone inside the castle helped Black get in, nonetheless the latter maintained that Lupin wasn’t trustworthy.

Harry Potter sitting in a classroom at Hogwarts in Harry Potter and the Prisoner of Azkaban.

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Snape Fills In For Remus Lupin

Harry, who arrives late for his Defense Against the Dark Arts lecture, is surprised to see Snape at the teacher’s desk. As punishment for arriving ten minutes late to class, Snape takes ten points from Gryffindor and tells him to sit down. His mouth twists in a smile when Harry asks him about Professor Lupin’s absence. He informs the class that their D.A.D.A. teacher is too ill to take a class. Snape continues with his lecture and remarks Lupin hasn’t left any record of the topics he’s covered so far. Hermione Granger (portrayed byEmma Watson inHarry Potter) reacts to his unfair criticism, saying, “Please, sir, we’ve done Boggarts, Red Caps, Kappas and Grindylows.” Agitated, Snape asks her to keep mum and tells the class that their D.A.D.A. professor has no organizational skills.

Snape says he would expect the first years to successfully deal with Red Caps and Grindylows, and jumps straightaway to the topic of Werewolves. He explains the rationale behind his decision, remarking that Lupin is hardly challenging his students and that they should be able to do much more. He scolds Hermione once again and when he yells at the class to turn their books to page 394, and gets bitter, and sullen looks in return. In the book-to-screen adaptation ofHarry Potter and the Prisoner of Azkaban, when Snape asks the students the difference between a Werewolf and an Animagus, it’s almost as if he’s dropping hints and helping them guess something that’s happening under their noses.

The Trio and Scabbers at the Shrieking Shack in Harry Potter and the Prisoner of Azkaban.

Professor Snape Set The Essay

Snape’s singular focus on the topic of Werewolves is a major clue that he has an ulterior motive. In J. K. Rowling’sHarry Potter and the Prisoner of Azkaban, he drops the line, “Well, well, well, I never thought I’d meet a third-year class who wouldn’t even recognize a Werewolf when they saw one.“After chiding rebuttals, more Gryffindor point deductions, and a detention, he sets the essay on how to recognize and kill Werewolves. This scene slightly differs in the screen adaptation ofHarry Potter and the Prisoner of Azkaban, as Snape demands the students write two rolls of parchment paper on Werewolf with a special focus on only recognizing it. In both versions, nonetheless, he demands the students submit this essay by Monday morning.

The question remains of why Snape chastises the class for not being able to recognize a Werewolf. The answer is plain and simple. As the thirdHarry Pottermovie- The Prisoner of Azkabannears its end, the trio is lured into theShrieking Shack by Sirius Black. Lupin arrives at the shack, disarms Harry, and lifts Black up, leading the Trio to think he’s allied with Black. Hermione decides right there and then to expose Lupin to her friends. She tells everybody the reason their teacher has been missing classes is that he’s a Werewolf. She also adds that she has known it ever since Professor Severus Snape set the essay. She’s the only one who’d completed the essay, and in doing so, she does right what Snape had intended for the class to find out through his assignment. Simply put, Professor Severus Snape didn’t trust Lupin, thought that he was helping Black into the Castle, and set the essay because he thought someone among the third years would be able to recognize he was a Werewolf.