In October 2020, HBO announced Paddy Considine (The Bourne Franchise) had been cast in the first significant role for House of the Dragon. Considine will play King Viserys Targaryen, whose reign spanned from 103-129 AC. Viserys was a wise ruler, but his love for his family kept him from seeing their true ambitions. His later years set up the events that would eventually lead to the Westerosi Civil War.
On Dec. 11, HBO confirmed three more major characters, rounding out the major Targaryen leads. Matt Smith, known for his run as the Eleventh Doctor on Doctor Who and Prince Philip on the first two seasons of The Crown, plays Daemon Targaryen. He is the younger brother to Viserys and uncle/husband of Viserys' eldest, Princess Rhaenyra. Daemon is a scheming warrior and is believed to have killed Rhaenyra's husband to marry her and cement a path to the throne.
Emma D'Arcy, best known for Hanna and Truth Seekers on Amazon, plays Princess Rhaenyra, Viserys' only surviving child from his first marriage and chosen heir. She has trained for the throne her entire life. Actor Milly Alcock (Reckoning) will play Young Princess Rhaenyra Targaryen.
Best known for A&E's Bates Motel, Olivia Cooke plays Alicent Hightower, Viserys' very ambitious second wife. She is famed for her beauty, but she possesses a keen political mind. On July 6, the production revealed it had cast Emily Carey (Wonder Woman) as Young Alicent Hightower.
Unfortunately for Rhaenyra, Viserys' rise to the throne a generation back cemented male primogeniture over firstborn women. When Viserys dies, many in the Small Council refuse to support her claim to the throne. Instead, they back Alicent's claim her young son, Aegon, should inherit, despite the late king's wishes.
With filming beginning in April, four more cast members were announced on Feb. 11, 2021, starting with Eve Best (Nurse Jackie) as Princess Rhaenys Velaryon; the woman passed over for the throne in favor of King Viserys a generation ago. Her loss is what sealed male primogeniture.
Her husband, Lord Corlys Velaryon, will be played by Steve Toussaint (Judge Dredd). Known as "The Sea Snake," this Essos adventurer is also from a Valyrian bloodline, though not a Targaryen. He's also super-rich, more so than even the Lannisters, and has his own navy. He and Rhaenys will be a powerful ally to whichever side they choose to back, and history suggests they would naturally sympathize with Rhaenyra.
Also joining the cast is Rhys Ifans (Harry Potter) as Otto Hightower, the Hand of the King and Alicent's father. Sonoya Mizuno (Crazy Rich Asians) has also been cast as Mysaria, a former enslaved Essos girl who has risen to become Daemon Targaryen's closest adviser.