'Game of Thrones' season 4 had already kicked off the casting race in advance of its panel at Comic-Con 2013, tapping Chilean actor Pedro Pascal in the fan-favorite role of Dornish prince Oberyn "The Red Viper" Martell, but it seems HBO had another major casting that just couldn't wait. 'Sherlock' star and writer Mark Gatiss has joined the epic HBO drama's fourth season, but as whom, you ask?

Well ... there's the rub. Entertainment Weekly revealed the casting, likely the first of many to be announced during the Friday panel, but HBO bosses are keeping Gatiss' role under lock and key. That could imply Gatiss' role is an original creation of HBO's adaptation, perhaps some consolidation of existing roles, or simply something to keep as a surprise in the coming days.

Outside of those familiar with George R.R. Martin’s books, relatively little is known of ‘Game of Thrones’ season 4, though we recently learned that previous season characters Lysa Arryn (Kate Dickie), Ser Alliser Thorne (Owen Teale), Rorge (Andy Beckwith) and Janos Slynt (Dominic Carter) would all return, among others. Yesterday saw the full list of writers and directors for the 2014 season emerge, revealing that season 2 "Blackwater" helmer Neil Marshall will return for the always-pivotal ninth episode.

We'll be on hand at the 'Game of Thrones' season 4 panel in San Diego this week, but what say you? Are you excited to see the 'Sherlock' / 'Doctor Who' writer-actor-producer take a mysterious role in season 4?

More From ScreenCrush