-New Zealand actor Joe Naufahu has been cast in a multiple arc role. They won't say who he's playing, but it's probably a Dothraki or someone else in Essos.
-Melanie Liburd has also been spotted on set. Popular speculation guesses that she's playing a red priestess named Kinvara in the Essos storyline.
-And then thanks to a super fan who crawled around and captured footage of Tower of Joy filming, Eddie Eyre, Luke Roberts, and Robert Aramayo can be seen as young Ned, Arthur Dayne, and Howland Reed. Not very clearly, mind you. And it was filmed in portrait because people never learn.
random fact: Joe Naufahu is my stepdad's nephew. never met him tho