Natalie Dormer, perhaps best known as Anne Boleyn in The Tudors, has been cast as Margaery Tyrell—only daughter of Lord Mace Tyrell and sister to the Knight of Flowers, Ser Loras—according to EW’s James Hibberd. There’s minor spoilers in Hibberd’s post, if you don’t want to know where Margaery fits into the story, don’t look.
Admittedly, Dormer is rather older than we had expected for this role, but of course she will doubtless play younger. Dormer is 29, while Finn Jones is 23—Loras is about a year older than Margaery in the novels, on the other hand.
Perhaps most notable about the news is the fact that Hibberd states she’s joining the regular cast of the series. The show—famous for having quite the large regular cast—seems likely to be expanding Margaery’s presence in the TV show, as compared to the novel, where she appears in what’s essentially two scenes.