Fandoms collide! Game of Thrones season 4 has cast a Sherlock actor and executive producer as production begins to roll.

Mark Gatiss has joined the fourth outing of Game of Thrones for an unspecified role, Entertainment Weekly has revealed.

While we’re bummed that further details haven’t been shared, we are excited by the fact that an actor from the beloved Sherlock is heading over to the equally (or more) beloved Game of Thrones for HBO.

Gatiss plays Mycroft on Sherlock. The show’s third series is expected to premiere in England by the end of this year. The actor will attend Sherlock’s first ever San Diego Comic-Con panel this weekend, and perhaps fans will grill him on this new Thrones role. It’ll be hard for fans to decide: “Should I ask about Sherlock?” or “Should I ask about Game of Thrones?”

Game of Thrones season 4 has cast one other role so far: The Red Viper. Yesterday we learned who would helm each episode of the highly-anticipated season based on George R.R. Martins’ A Song of Ice and Fire book series. The show began shooting last Monday.

The most popular fan theory is that Gatiss will be playing Mace Tyrell, the father of Joffrey’s wife-to-be Margaery.

However, considering that season 3 ended with Yara Greyjoy setting out to save her brother Theon, the show might also be looking to focus more on the Greyjoys in season 4. In that case, we think Gatiss would make a great Aeron (Damphair) or Euron!

Do you have any predictions concerning who Gatiss will play?

Thanks, EW.