J.K. Rowling is a little disappointed she wasn't able to keep Johnny Depp's casting in Fantastic Beasts and Where to Find Them a secret for just a little bit longer.

ET spoke to the author at the New York City premiere of the highly anticipated film, in which she talked about Depp's headline-making casting as Gellert Grindelwald in the franchise. Depp has a cameo in Fantastic Beasts, and will appear more heavily in its sequel.

"It was enormously hard, and I can't believe we kept that under wraps until barely a week from the movie, so I suppose we kind of wanted to give people a surprise, that reveal," she said about news of the cameo breaking before the movie even came out. "Would've been nice, but I think we're pleased we got this far."

WATCH: Spoiler Alert! Johnny Depp's 'Fantastic Beasts' Role Revealed

Aside from Depp, Rowling gushed about Oscar-winner Eddie Redmayne, who plays Newt Scamander in the film.

"He's one of the best actors in the world but he's got a real feeling for who Newt is," she said. "Newt's a ... he's a guy who in some ways, he's immensely confident and strong, and in other ways he's very uncomfortable in his own skin. He's British."

Rowling also gave us a hint about what fans can expect from the upcoming four films in the franchise.

"Well, I think that it's kind of an open secret now that we're not just telling the story of the creation of that little book, Fantastic Beasts," she noted. "This whole story will span 19 years, so hardcore Potter fans I think will know what that means. If we're going from 1926 to 1945, they'll know what it's about."

Fantastic Beasts hits theaters on Nov. 18.

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Director David Yates recently defended Depp's casting in an interview with The Leaky Cauldron.

"The whole principle of casting the movie was go with the best actor. Go for the most inspired, interesting, right fit for that character," he said of casting the key role of Grindelwald, one of the most dangerous dark wizards of all time. "And as we approached Grindelwald we thought, 'Who's going to take this in an interesting direction?'"

"We were excited about seeing what he would do with this guy, the character. He's fearless; he's imaginative; he's ambitious," Yates continued about Depp. "We thought he would do something fun and special. So we went for him, purely on that selfish basis. We don’t care if he’s famous or not famous. We just know he's interesting."

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