Why Robin Williams lost out on the role of Remus Lupin in ‘Harry Potter’

David Thewlis brought Harry Potter's beloved Remus Lupin to life but did you know that another famous actor had expressed interest in playing the role before Thewlis bagged the part?

That's right! Chris Columbus, who directed Harry Potter and the Philosopher's Stone, the first film of the franchise, marked its 20-year anniversary on an interview with Total Film, per GamesRadar. During his conversation, he revealed that Robin Williams wanted to play Lupin, a werewolf who is first introduced in Harry Potter and the Prisoner of Azkaban, the third book and film in the franchise.

Columbus shared that Williams lost out on the role because casting him would have broken a rule to avoid casting any American actors in an effort to "maintain a degree of authenticity.” Columbus, who is American, recalled, "I had a conversation with Robin Williams, who wanted to play Lupin. It was very difficult for me to say, 'It's all British. There's nothing I can do.' "

Columbus directed both the first and second Harry Potter films, while Alfonso Cuarón stepped in for the third, which also featured Thewlis' first appearance as Lupin in 2004. The actor went on to play the role up until his final appearance in 2011.

Casting director Janet Hirshenson had detailed the British actors rule in a 2016 interview with HuffPost, relaying a similar story about Williams' hopes to join the franchise. "Robin [Williams] had called [Chris Columbus] because he really wanted to be in the movie, but it was a British-only edict, and once he said no to Robin, he wasn't going to say yes to anybody else,” she shared.

Williams, who died in August 2014, briefly mentioned the Harry Potter casting rule in a 2001 interview with The New York Post. "There were a couple of parts I would have wanted to play, but there was a ban on [using] American actors," he said, per The Independent, before adding, "Maybe one day. Say if [Harry] goes to Yale and becomes president."