A star of the 'Harry Potter' spinoff thought she bombed her audition

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"Fantastic Beasts and Where to Find Them" star Katherine Waterson didn't expect to be cast in the movie.

When Warner Bros. offered her an audition for the "Harry Potter" spinoff, Waterson thought "that's way above my pay grade, and it's not going to happen," she told Entertainment Weekly. But she auditioned anyway and thought "it really couldn't have gone worse."

Here's how she described the experience to EW:

"Well, the casting director had just eaten a lot of cake, and he said, 'I'm sorry, I just ate a lot of cake,' and then he had like indigestion during my audition, and he didn't seem to be watching me. So I thought, 'Oh, I'm boring him.' So I was very surprised to hear that they wanted to put me through the ringer and do a test, and then that's just a nightmare because then you're really attached by that point and you're fantasizing and thinking about the character and why you're the only person who can play her because of whatever reasons you've dreamed up. You start wanting to play the scenes."

To her happy surprise, Waterson got the role. She'll be playing Porpetina Goldstein, an ex-auror at the Magical Congress of the United States of America (MACUSA), now demoted to a desk job. She and her sister cross paths with Newt Scamander, the main character in the movie, in 1920s New York.

For her role, Waterson - who's been in "Inherent Vice" and "Steve Jobs" - got to determine a lot about her character, like her wand. We'll see her performance when "Fantastic Beasts" is in theaters on November 19.