Olivia Cooke is an English actress best known for starring as Emma Decody on Bates Motel for five seasons. She also played Becky Sharp in the 2018 ITV mini-series version of Vanity Fair. She also played one of the leads in Steven Spielberg's Ready Player One. Other roles include Modern Love, Blackout, Sound of Metal, The Quiet Ones and the frankly superb credit "Voice of the Loch Ness Monster" on Axe Cop.
Alicent Hightower is the intelligent and cunning daughter of Lord Otto Hightower, Lord of Oldtown and the Hand of the King. The ambitious Lord Otto advises his daughter to help comfort the widowed king and befriend his daughter Princess Rhaenyra, only a few years younger.
Emma D'Arcy is an English actress who started her career by making waves on stage, particularly for her role alongside Ben Whishaw in Against. Regular and recurring roles have followed on Wanderlust, Wild Bill, Hanna and, most recently, a more complex-than-it-first-appears role on the Nick Frost semi-comedy Truth Seekers.
Princess Rhaenyra Targaryen is the only child of King Viserys I Targaryen. Male primogeniture is the preferred method of inheritance in the Seven Kingdoms of Westeros, but King Viserys has raised Princess Rhaenyra to be his heir, teaching her the art of rule and statecraft, inviting her to small council meetings and learning military strategy. Despite some grumblings from traditionalists, the king's decision has been accepted...whilst he still lives. Like many of the Targaryens of this time, Rhaenyra is a dragon-rider. Her dragon is called Syrax.
Matt Smith is an accomplished English actor best-known for his starring role on Doctor Who. He played the Eleventh Doctor for four years between 2010 and 2013, winning two National Television Awards for his performance. He remains the youngest actor to ever play the role. His other performances include Jim Taylor in the BBC adaptations of Philip Pullman's Sally Lockhart Mysteries, Danny in Party Animals, Skynet in Terminator: Genisys (2015) and Parson Collins in Pride and Prejudice and Zombies (2016). In 2016-17 he played Prince Philip in the first two seasons of The Crown, for which he was nominated for an Emmy.
Prince Daemon Targaryen is the younger brother of King Viserys, both grandsons of the Old King, Jaehaerys the Conciliator. Whilst Viserys is diplomatic, restrained and thoughtful, Daemon is fiery and prone to action. He is intelligent, but also hot-tempered and passionate, who sometimes feels his brother acts too little, too late in response to provocations from the Seven Kingdoms' enemies. For his part, the king despairs of what he sees as his brother's inability to see the big picture and restrain himself in the short-term for future long-term benefits. Daemon would be his brother's heir if he had not named Rhaenyra, something he struggles with despite his general affection for his his niece. Daemon rides the immense dragon Caraxes, a fearsome beast and the largest of the Targaryen dragons apart from Vhagar (the only surviving dragon of Aegon the Conqueror's original three).
Based on the age of the actors being cast and the information released by HBO, it sounds like the series will start some time before the beginning of the Dance of Dragons, maybe as much as twenty years earlier (roughly 190 years before the events of Game of Thrones), when King Viserys is still relatively hale and trying to keep his brother under control whilst also training his daughter to follow him onto the Iron Throne. I suspect the timeline will either be compressed in the series or there'll be some hefty timeskips to take us into the Dance of Dragons by the end of the first season.
Update: The mostly-reliable Redanian Intelligence has indicated from casting material that the show begins in 105 AC with the death of Queen Aemma, and thus with King Viserys recovering from the shock of her death and Princess Rhaenyra from the loss of her mother.
Deadline is also reporting that Danny Sapani (Black Panther, The Last Jedi) is being considered for the role of Corlys Velaryon, the "Sea Snake," a superior sailor and naval commander who has become immensely wealthy by sailing to the ends of the earth and bringing him immense treasure.
House of the Dragon is currently in pre-production and is expected to start shooting in England in early 2021. It should air on HBO in early 2022.