Game of Thrones actress Maisie Williams, who plays free-spirited Arya Stark in the show, will join the cast of Doctor Who in an upcoming episode airing later this year. Details about who she will play is currently unknown, though Steven Moffat did have something to say about her casting;
“We’re thrilled to have Maisie Williams joining us on Doctor Who. It’s not possible to say too much about who or what she’s playing, but she is going to challenge the Doctor in very unexpected ways. This time he might just be out of his depth, and we know Maisie is going to give him exactly the right sort of hell.”
So it looks like Maisie will be playing a role not too dissimilar to that of Arya Stark; a cheeky, headstrong girl. She will join Peter Capaldi and Jenna Coleman on screen, with comedian Rufus Hound who was also announced as a guest star.