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The “Game of Thrones” prequel pilot at HBO is starting to round out its cast.

Josh Whitehouse has joined the project, though the exact nature of his role is being kept under wraps. Whitehouse is now the second actor officially cast in the pilot, with Naomi Watts having been announced as one of the leads on Tuesday.

Whitehouse recently starred in the BBC series “Poldark” and will star in the remake of “Valley Girl” currently in the works. He is repped by CAA, United Agents, Untitled Entertainment, and Gang, Tyre, Ramer & Brown.

Taking place thousands of years before the events of “Game of Thrones,” the new show chronicles the world’s descent from the golden Age of Heroes into its darkest hour. And only one thing is for sure: from the horrifying secrets of Westeros’ history to the true origin of the white walkers, the mysteries of the East to the Starks of legend–it’s not the story we think we know.

Jane Goldman co-created the project with George R.R. Martin, whose “A Song of Ice and Fire” novel series serves as the basis for “Game of Thrones.” Goldman and Martin wrote the story, with Goldman writing the teleplay. She will also serve as showrunner. She and Martin will executive produce along with Daniel Zelman, James Farrell, Jim Danger Gray, and “Game of Thrones” executive producer Vince Gerardis. Chris Symes will co-executive produce.

Deadline first reported Whitehouse’s casting.