Went to look if anything had been posted recently, couldn't find anything, but three new cast members were announced today, joining Paddy Considine, who is playing King Viserys Targaeryn.

Olivia Cooke as Alicent Hightower. She's the daughter of Otto Hightower, the Hand of the King, and the most comely woman in the Seven Kingdoms. She was raised in the Red Keep, close to the king and his innermost circle; she possesses both a courtly grace and a keen political acumen.
Emma D'Arcy as Princess Rhaenyra Targaryen. The king's first-born child. She is of pure Valyrian blood, and she is a dragonrider. Many would say that Rhaenyra was born with everythingbut she was not born a man.
Matt Smith as Prince Daemon Targaryen. The younger brother of King Viserys and heir to the throne, Daemon is a peerless warrior and a dragonrider who possesses the true blood of the dragon. But it is said that whenever a Targaryen is born, the gods toss a coin in the air

Olivia Cooke as Alicent Hightower. She's the daughter of Otto Hightower, the Hand of the King, and the most comely woman in the Seven Kingdoms. She was raised in the Red Keep, close to the king and his innermost circle; she possesses both a courtly grace and a keen political acumen.
Emma D'Arcy as Princess Rhaenyra Targaryen. The king's first-born child. She is of pure Valyrian blood, and she is a dragonrider. Many would say that Rhaenyra was born with everythingbut she was not born a man.
Matt Smith as Prince Daemon Targaryen. The younger brother of King Viserys and heir to the throne, Daemon is a peerless warrior and a dragonrider who possesses the true blood of the dragon. But it is said that whenever a Targaryen is born, the gods toss a coin in the air