Astrid Berges-Frisbey (I Origins) has signed on to star in Warner Bros.' Knights of the Roundtable: King Arthur as Guinevere. The actress beat out a number of notable actresses who were vying for the role, including Elizabeth Olsen, Felicity Jones and Alicia Vikander.Sons of Anarchy star Charlie Hunnam signed on last month to play the title character, with Idris Elba joining the cast in July as Bedivere, the right-hand man to Arthur's father who teaches the young Knight how to fight.