As OG Potterheads will constantly remind you, the books came before the films.
But in order for the first movie to get made, it needed the go-ahead from Harry Potter author JK Rowling.
The 58-year-old - who has been criticised for her views on transgender rights - made one big casting rule, which ended up getting broken a fair few times.
Director of the first two Harry Potter movies, Chris Columbus, claims he was told that the cast of the films must be '100 percent British'.
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This might not sound like the craziest of requests, until you realise that Robin Williams called the director and asked if he could be in Harry Potter as Hagrid, in which the answer would have been 'no'.
Of course, the late great Robbie Coltrane did an incredible job, so we won't begrudge anyone for that.
Although, there were, in fact, several non-British actors in the Harry Potter movies as a decent number of the cast were Irish, including both Dumbledore actors Richard Harris and Michael Gambon.
Still, Rowling's rule was really more concerned with her story about the boy wizard not being turned into an American-dominated tale by movie studios.
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With that in mind, there were only two American actors actually got cast in the Harry Potter films, though you won't have heard their voices when you watched the movies.
One of them was director Chris Columbus' own daughter Eleanor, who appeared as Susan Bones, a girl who is sorted into Hufflepuff by the Sorting Hat during Harry Potter and the Philosopher's Stone.
She doesn't have a speaking part in the film as Harry (Daniel Radcliffe) is far too preoccupied about his scar hurting and mistakenly thinking the cause is the death-glare he's getting from Severus Snape (Alan Rickman).
While Harry is too engrossed in his own personal drama, Susan Bones gets sorted into Hufflepuff and then it's Ron's (Rupert Grint) turn to don the talking hat.
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In a later appearance in Harry Potter and the Deathly Hallows, she was played by Emma Jayne-Corboz.
The other American who appears in the Harry Potter films also shows up in Philosopher's Stone and that's Verne Troyer, who provided the physical acting for Gringotts Bank goblin Griphook.
However, Warwick Davis provided the voice for Griphook and later played the role when the goblin banker returned for the final two films.
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One could argue that there are technically three American actors in the Harry Potter films since Zoë Wanamaker was born in New York City, but when she was a child, her father was blacklisted due to his communist views.
He'd been over in England for a play at the time and upon hearing of his blacklisting decided to stay in the UK, meaning his daughter has British citizenship and has spent the vast majority of her life in the UK.
Featured Image Credit: Warner Bros./Mike Marsland/WireImageTopics: Harry Potter, JK Rowling