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The star, who bagged an Oscar-nomination earlier this year, will play the part of former drag queen Hugo in the movie – and, frankly, it's something we just can't wait to see.
The hit musical is based on the true story of Jamie Campbell, and follows the titular character who lives in a council estate in Sheffield as he overcomes prejudice to become a drag queen.
Shop Now Everybody's Talking About Jamie (Apollo Theatre, various shows, June 17-Jan 25)
Hugo acts as a mentor to the youngster – played by newcomer Max Harwood. Catastophe star Sharon Horgan will play teacher Miss Hedge, while Happy Valley's Sarah Lancashire will take on the role of Jamie's mother, Margaret.
Grant played Dr Simeon/The Great Intelligence on Doctor Who, and also had a small role on Game of Thrones as actor Izembaro in season 6.
The actor opened up earlier this year about working on the movie, explaining the lengths that Disney goes to to keep the plot secret.
"It's extraordinary," he said. "They don't give you a script, you have to go to a room where there are bodyguards outside, closed-circuit television cameras, and it's printed on crimson pages so you can't photograph it.
"You have to read it in there, and then leave."
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