David Thewlis played Professor Remus Lupin in Harry Potter and the Prisoner of Azkaban.
Lupin joins Hogwarts as the Defence Against the Dark Arts teacher in the third film. He is presented as an unusually effective and sympathetic teacher, while the story later reveals his connection to Sirius Black and James Potter.
Thewlis returned as Lupin in later films, including Harry Potter and the Order of the Phoenix, Harry Potter and the Half-Blood Prince, and Harry Potter and the Deathly Hallows. His character’s werewolf identity becomes important to the plot and to the history of Harry’s parents.
Other adult performers have distinct roles in the same film: Alan Rickman played Severus Snape, Brendan Gleeson played Alastor Moody in later films, and Kenneth Branagh played Gilderoy Lockhart in the second film.