In a 20th anniversary retrospective on the "Harry Potter" film series, Total Film magazine interviewed director Chris Columbus about the casting of the film. He revealed that Robin Williams was very interested in one of the biggest roles in the "Harry Potter" franchise: that of Remus Lupin, a wizard who was the Defense Against the Dark Arts professor at Hogwarts (and a werewolf).
According to Columbus, the role went to David Thewlis instead of Williams for a simple reason: authenticity. Columbus had made a rule that he wouldn't cast any American actors, in keeping with the British roots of the novels.
"I had a conversation with Robin Williams, who wanted to play Lupin," Columbus told Total Film. "It was very difficult for me to say, ‘It’s all British. There’s nothing I can do.'"aside">
Williams was also in the news last week after a lifelike impersonation of him went viral. It has since been removed from creator Jamie Costa's YouTube channel, but can be seen here.More Robin Williams
Source : https://www.sfgate.com/streaming/article/robin-williams-harry-potter-casting-16542878.php570