Josh Whitehouse is joining the cast of the "Game of Thrones" prequel. Deadline confirmed the 28-year-old "Poldark" actor will star with Naomi Watts in the new HBO pilot. The Hollywood Reporter said the British actor will play a key role in the series but not the male lead. Details about his character are being kept under wraps. Whitehouse confirmed his casting in an Instagram post Wednesday. "So I've been holding on to some very big news and it's been killing me - but here you go #gameofthrones," he wrote. News broke Tuesday that Watts will play a charismatic socialite with a secret in the prequel. The new show was the first of several "successor series" to be picked up to pilot and is set thousands of years before the events of "Game of Thrones." "Game of Thrones" is based on the George R.R. Martin book series "A Song of Ice and Fire." Martin confirmed Watts' casting in a blog post and referred to the prequel by the title "The Long Night." Whitehouse plays Hugh Armitage on "Poldark," which completed a fourth season on BBC One in July.