In a surprising turn of events, Mark Gatiss is not Mace Tyrell. I suppose now he will probably be either a Greyjoy or a Frey. Considering he is also doing Doctor Who and Sherlock, it's safe to assume that he'll probably play a one-off character, like Merrett Frey or Randyll Tarly. Giddy.
British actor Roger Ashton-Griffiths has been cast as Dame Diana Rigg's son, the bumbling fool, Mace Tyrell. This gives us another reason why the casting choice of the show is outstanding. He's a bit more than I expected as Mace, but he may be able to easily pull it off. He's not getting that many scenes though, that's for sure.