The English actor, 50, is set to appear as "a charismatic socialite hiding a dark secret" in the prequel's pilot, Variety reported on Tuesday. Further details of her character's backstory have yet to be revealed.
Screenwriter Jane Goldman, who co-wrote Kick-Ass with Matthew Vaughn, is behind the prequel, which takes place thousands of years before the events told in Game of Thrones. The new project will detail how the world fell into darkness from the prosperous Age of Heroes.
Watts has also been cast to play Gretchen Carlson in Showtime's limited series about the late Fox News CEO and disgraced television executive Roger Ailes.
Goldman worked on the prequel's story with author George RR Martin, whose best-selling novels published as part of the series A Song of Ice and Fire were adapted into the cult Game of Thrones. The screenwriter will work as the prequel's showrunner. Goldman and Martin will both serve as executive producers.
It was first revealed in June this year that HBO had ordered a pilot for the prequel from Goldman.
HBO currently has five HBO-related projects in the works, according to Variety. Details pertaining to each project have been kept secret, though Martin has said that the mysterious new ventures are prequels, not spin-offs, and that none of the main actors from Game of Thrones are set to appear in them.
Game of Thrones, which aired for the first time in 2011, is scheduled to conclude with its eighth season in 2019.
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