'Game of Thrones' Prequel: Meet the Director & New Cast Members (PHOTOS)

The still-untitled Game of Thrones prequel is making fast progress.

The HBO pilot has added many diverse cast members: Toby Regbo (Reign, Fantastic Beasts: The Crimes of Grindelwald), Ivanno Jeremiah (Humans, Black Mirror) and Georgie Henley (The Chronicles of Narnia, The Spanish Princess), Naomi Ackie (Star Wars: Episode IX), Denise Gough (The Fall), Jamie Campbell Bower (Twilight, Camelot), Denise Gough, (People Places and Things and Angels In America), Sheila Atim (Harlots) and Alex Sharp (Broadway’s The Curious Incident of the Dog in the Night-Time, How to Talk to Girls at Parties) among others.

The newest additions include Marquis Rodriguez (When They See Us), John Simm (Strangers), Richard McCabe (Philip K. Dick's Electric Dreams), John Heffernan (Dracula) and Dixie Egerickx (The Secret Garden).

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They are joining previously announced cast members Naomi Watts, a "charismatic socialite hiding a dark secret," and Josh Whitehouse, but all their character descriptions are being kept under wraps.

Also announced is the series' director, S.J. Clarkson. Her impressive list of work includes episodes of Orange Is the New Black, Dexter, Bates Motel, The Defenders and Jessica Jones. She joins showrunner Jane Goldman and will executive produce as well.

The official series logline reads, “The series chronicles the world’s descent from the golden Age of Heroes into its darkest hour. And only one thing is for sure: from the horrifying secrets of Westeros’ history to the true origin of the white walkers, the mysteries of the East to the Starks of legend — it’s not the story we think we know."

Click through the gallery above to meet the cast and director.