Game of Thrones

HBO

The world of the "Game of Thrones" prequel pilot just keeps expanding.

HBO has added five actors in series regular roles: 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").

The pilot is written by Jane Goldman with a story from George R.R. Martin,. The untitled prequel takes place thousands of years before the events of Game of Thrones. It 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.

The new cast members join an already impressive list that includes Naomi Watts, Miranda Richardson, and Josh Whitehouse.

The prequel has been in the works for awhile now, having survived a process where HBO put a handful of "Game of Thrones" spinoffs into development.

The flagship series premieres its eighth and final season April 14.