In the show, Littlefinger is an adviser to King Robert who takes advantage of those in power to fit his own agenda.
Gillen played Councilman Thomas "Tommy" Carcetti on the network's acclaimed The Wire.
Game of Thrones is preparing to start production in Northern Ireland later this month. This will include reshooting the majority of the pilot following the network recasting several roles. HBO wants the drama on the air as early as next spring.