Extracted from Harry Potter: The Creature Vault by Jody Revenson
The true shape of a Boggart is unknown, as a Boggart changes its shape to that of the viewer’s most horrible fear. In Harry Potter and the Prisoner of Azkaban, Defence Against the Dark Arts professor Remus Lupin instructs his third year students how to cast the Riddikulus charm to repel such a creature. Lupin also uses a Boggart in a private lesson in the same film to teach Harry Potter how to cast the Patronus charm in defence against a Dementor – Harry’s worst fear.
Neville Longbottom is first to practice the Riddikulus charm in Harry Potter and the Prisoner of Azkaban, redressing Professor Snape in grandmother Longbottom’s bird-topped hat and tweedy skirt ensemble. Ron imagines roller skates on the eight legs of an Acromantula. Parvati transforms a cobra into a jack-in-the-box. But when Harry sees a Dementor, Lupin intercedes. Lupin’s worst fear is a full moon creeping out of a shadowy sky, which he turns into a deflating white balloon, all effects created in the computer.
Although an actual Boggart is never seen, the visual effects designers still needed to create the transition from the scary version to the laughable version, which they interpreted as a swirling tornado of the Boggart’s fearful and funny forms.