Casting rumors about the third season of "Game of Thrones" have been swirling for weeks, and HBO finally put fans out of their speculation misery at Comic-Con by confirming the identities of more than a dozen actors who'll be joining the series in Season 3.Diana Rigg of the classic British series "The Avengers" joins "Game of Thrones" as Lady Olenna Tyrell, "The Queen of Thorns." She is the grandmother of the aristocrats Loras and Margaery Tyrell and the matriarch of the powerful Tyrell clan.