On Saturday's episode, "The Girl Who Died," Williams plays a girl named Ashildre, who is one of several Vikings tasked with defending themselves against the deadliest army in the galaxy. Luckily, the Game of Thrones star has seen her share of combat on the HBO drama.
And, judging by the reaction on social media when it was announced that Williams had been recruited to star alongside The Doctor (Peter Capaldi), it's a safe bet that several Game of Thrones fans will be tuning in to "The Girl Who Died," which is the first of a two-episode storyline.
"When you cross over to a different geeky fandom world, everyone's like, 'This is insane!'" Williams tells the BBC. "It's so nice to know that there is a bit of hype around it before it comes out, but it also is a lot of pressure too. ... I hope that it lives up to everyone's expectations, and I can't wait to see how people like it."
Get a first look at Williams' appearance here:
Doctor Who airs Saturdays at 9/8c on BBC America.