'Game of Thrones' casts 'Sherlock' actor in season 4

This is the sort of casting us geeky TV bloggers dream about: A Game of Thrones, Doctor Who, Sherlock combo!

Mark Gatiss will appear in the fourth season of HBO’s Thrones. The British writer-producer-actor has written Doctor Who novels and episodes of the BBC cult classic, as well as acted on the long-running series (since 2007, he’s credited to three different roles on IMDB). In addition, Gatiss is the co-creator and co-star of another Brit-based favorite, Sherlock, where he plays Holmes’ dryly mysterious brother Mycroft. Now he’s booked on HBO’s fantasy hit next year.

On Thrones he will play …

Well, that’s something we cannot reveal. The Thrones team is keeping Gatiss’ role quiet. Maybe showrunners David Benioff and Dan Weiss will say more at their Comic-Con panel on Friday? Also, Sherlock is making its first-ever SDCC panel appearance on Thursday (I’m moderating) so maybe Gatiss will spill something then. EW.com will bring you full coverage of both events, plus we will stream our own interviews with the Thrones and Sherlock producers on the EW.com homepage.

While Thrones added numerous new roles for its second and third seasons, population growth is slowing considerable for season four (the show already has a larger cast than any drama on television). Gatiss joins fellow season 4 newcomer Pedro Pascal, who will play the key role of Prince Oberyn “The Red Viper” Martell. Also, yesterday we revealed the list of directors for season 4.