Over the past month, Mark Gatiss, Joseph Gatt, and Michael Huisman have all been cast in unspecified roles for Game of Thrones Season 4.

We’ve seen a lot of speculation on the issue, but The Mary Sue has some interesting theories:
It’s a fair bet that Gatt will play Styr, Magnar of Thenn, since we know he’s definitely showing up in season four and Gatt looks, well, Styr-y. My guess for Gatiss is Merrett Frey; if you haven’t read the books, do not Google him, as pretty much everything about his character constitutes a major spoiler. Mace Tyrell and Oberyn Martell have already been cast …
If we are getting book four characters one of the Martells is also a possibility, though the show has cast persons of color for the two of them we know we’re getting so far (Oberyn and Ellaria Sand). Might we be seeing Ser Arys Oakheart, the Kingsguard member who protects Myrcella in Dorne? Huisman visually matches Oakheart for me more than any other character, but that doesn’t necessarily mean anything.