Update; Game of Thrones season three will premiere March 31, 2013. The announcement was made at the show's Comic-con panel.
The cast of Game of Thrones is expanding. HBO has announced that Diana Rigg will play Lady Olenna Tyrell, known as The Queen of Thorns. She is the powerful matriarch of the Tyrell family, grandmother of Margaery and Loras Tyrell. Other new cast members include Mackenzie Crook, Clive Russell, Nathalie Emannuel, Kerry Ingram, Paul Kaye, Thomas Brodie-Sangster, Ellie Kendirck, Richard dormer, Kristofer Hivju, Phillip McGinley, Tara Fitzgerald, Tobaias Menzies and Anton Lesser. You can read more about their characters here.