'Game of Thrones' Prequel Casts One of Its Male Leads

The Game of Thrones prequel has enlisted another actor: Josh Whitehouse.

Whitehouse, best known for his work as Hugh Armitage on Masterpiece's Poldark, as well as his upcoming turn in the remake of Valley Girl playing the role originated by Nicolas Cage, will be joining the cast of HBO's upcoming successor show to Game of Thrones. There are no current details on the character he's playing; it's described as a key role, though not the central male lead.

News of Whitehouse's involvement follows the official casting of Twin Peaks: The Return star Naomi Watts. As with Whitehouse, details on Watts' character are under wraps, though slightly more information is known: She's playing a charismatic socialite hiding a dark secret. Some Thrones fans have speculated that Watts could play an ancestor of the Lannister family, though it's nothing more than conjecture at this time.

The Game of Thrones successor show is created by Jane Goldman and A Song of Ice and Fire author George R.R. Martin. The pilot, which was ordered by HBO in June, is based on a story from Goldman and Martin, with the teleplay credited to Goldman. She also will serve as showrunner.

In addition to Goldman and Martin, executive producers on the project include current Game of Thrones producer Vince Gerardis, as well as Damages and Bloodline co-creator Daniel Zelman and Jim Danger Gray. Chris Symes will serve as a co-exec producer.

The untitled series takes place thousands of years before the events of Game of Thrones, and will chronicle the end of the Age of Heroes, a period in Westeros history marked by legendary figures like "Bran the Builder" and "Lann the Clever," originators of the Stark and Lannister families. Little else is known, which is just as HBO would like it for now, based on the official logline: "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."

Goldman and Martin's Game of Thrones prequel is the first of several potential series set in the world of Westeros to move forward at HBO beyond the script phase. In May 2017, a multitude of writers were revealed by the pay cabler to be working on what Martin himself has described as "successor shows." Those writers include Goldman, Max Borenstein (Kong: Skull Island), Brian Helgeland (Legend), Carly Wray (Westworld) and Game of Thrones veteran Bryan CogmanGame of Thrones creators and showrunners David Benioff and Dan Weiss are not involved in the successor show effort, focusing instead on the forthcoming final season of Game of Thrones after more than a decade spent wandering the world of the Seven Kingdoms.

Speaking previously with The Hollywood Reporter, HBO programming president Casey Bloys said that any Game of Thrones successors, if greenlighted, would not air "until at least a year after the final season." The exec also added that he did not expect every one of the scripts to move forward beyond the page.

Follow THR.com/GameOfThrones for more coverage.

Deadline was first to report the casting.