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As we all know, the Harry Potter books were a huge international hit, spawning a similarly massive movie series.
But author J.K. Rowling was adamant that, while the franchise might have global appeal, the films shouldn't stray from their British origins.
As such, she insisted that all major cast members must be British or Irish - meaning the daughter of Harry Potter and the Philosopher's Stone director Chris Columbus couldn't utter a line in the 2001 movie.
Eleanor Columbus - who, like her dad, is American - played the part of Susan Bones, who was one of the first in Harry's class to pop on the old sorting hat.
Eleanor also starred in Harry Potter and the Chamber of Secrets, with her dad returning to direct.
Speaking to Insider, Home Alone director Columbus recalled: "Jo Rowling and myself said to each other when we first met, 'Look, we want this cast to be 100 percent British.'
"And by the way, I stuck to that. She [Eleanor] worked about 80 days, but she never spoke because you know the rule was if you're not British, you can't speak."
A major upshot of this was that Robin Williams was prohibited from being in the films.
The late great had shown an interest in playing Hagrid and Professor Remus Lupin.
Speaking to the New York Post in 2001, Williams said: "There were a couple of parts I would have wanted to play, but there was a ban on American actors.
"Maybe one day, say if [Harry] goes to Yale and becomes president."
Columbus said it was hard turning down the much-loved actor.
Speaking to Total Film last month, he said: "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.'
"Robin would have been brilliant. It would have been a different interpretation - I thought David Thewlis was great - but Robin would have been brilliant."
Casting director Janet Hirshenson said that once they'd managed to turn away Williams, there was no doubt they'd be sticking to the Brits only rule.
Speaking to the HuffPost in 2016, she said: "Robin [Williams] had called [director 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, that's for sure. It couldn't be."
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