'Game of Thrones' Season 4 Spoilers, Casting News: 'Sherlock' Showrunner & Actor Mark Gatiss Joins HBO Series, Role Kept Secret

HBO's "Game of Thrones" has just added a new cast member to its upcoming foruth season, which is currently filming. Mark Gatiss has joined the series as a yet-to-be named George R.R. Martin character for season four. Showrunners David Benioff and D.B. Weiss are keeping quiet about Gatiss' role, but it could easily be revealed at this summer's Comic Con panel.

Gatiss is a very familiar face over at BBC. He has had many roles in hit shows including "Doctor Who" and "Sherlock." Gatiss is also a seasoned writer, having written a number of "Doctor Who" episodes and serving as showrunner over at "Sherlock," of which he is also a co-creator. Fans familiar with Gatiss' work will note that the actor has incredible range and will surely fit well into season four of "Game of Thrones."

What character do you think Gatiss will play in season four?

Last month, Pedro Pascal was cast in the role of Prince Oberyn "The Red Viper" Martell.

"This was a tough one," explained showrunners Benioff and Weiss about the casting. "The Red Viper is sexy and charming, yet believably dangerous; intensely likable, yet driven by hate. The boys love him, the girls love him, and he loves them all back. Unless your last name is Lannister. We found a fellow who can handle the job description and make it seem effortless. He wasn't easy to find and he won't be easy to stop."

The fourth season of "Game of Thrones" will air in the spring of 2014 with production already underway.