George has posted that fans have guessed some of his clues, but one remains elusive.
As per our previous post, our endorsement of the guess of Derek Halligan for Ser Alliser Thorne is correct; good job, Mormegil99! You can learn more about Halligan at Ulster Actors. The other clue, the lord who never had lines in the books, will be played by Sir John Standing, who played one of the leads in the adaption of GRRM’s novella, Nightflyers, more than 20 years ago.
GRRM promises many more roles to be announced shortly—we expect some news today—and hints that “maybe even a big one that you have all been eager for” will also be revealed in time.
Below is a video of Sir John conducting a public reading: