The Harry Potter movies made household names out of so many of its cast, including Daniel Radcliffe, Rupert Grint, Emma Watson and Tom Felton.
Others, like Alan Rickman, Maggie Smith, Julie Walters and Helena Bonham Carter, were already huge stars.
And as time has gone on, another type of Harry Potter star has emerged - basically, those who we didn't realise were in the films, usually in small roles or as extras, until they became famous for something else.
Watch: Derek Hough on being an extra in Harry Potter and the Philosopher's Stone. Post continues below video.
It's happened a lot over the years, which is probably not that surprising given how large scale the franchise was. There needed to be hundreds of other witches, wizards and muggles wandering around and for some, Hogwarts marked the very beginning of their acting career.
Here are 10 celebrities you may not have noticed were in Harry Potter.