"Under The Lake," the most recent episode of Doctor Who, not only made ghosts more terrifying than ever, but also introduced the first deaf actress to appear on the series.

In a new Doctor Who Extra, the BBC introduces us to that actress, Sophie Stone, as well as Zaqi Ismail, who also appeared in "Under The Lake," as they discuss their characters in the show's current two-part story arc that wraps up this weekend.

Stone is no stranger to being first at anything: she was also the first deaf person to study at the Royal Academy of Dramatic Arts. That eventually led to her appearance on Doctor Who. In this clip, though, Stone talks about what drew her to Cass, the character she plays, and praises the episode's writing by Toby Whithouse.

"The fact that he's written for a deaf character that isn't vulnerable, who isn't a victim, or typecasted as someone who can't – she was definitely a can-do girl," says Stone in the video.

Stone also speaks about the challenge of acting without her hearing aid, and how she often used visual cues to keep her in the moment as she portrayed Cass during the filming of the episode.

Peter Capaldi, who plays the 12th Doctor, also discusses the Doctor's relationship with Stone's character, acknowledging that the Doctor immediately understands that exempting himself, she's the smartest person on the ship that he and Clara find themselves exploring in the episode, which features ghost-like beings chasing down the crew. Actress Jenna Coleman, who portrays Clara, also praised Stone's work on the series.

Finally, those who've watched "Under The Lake," understand immediately that there's a relationship between Cass and Lunn (Ismail). Coleman hints at a love between the two.

In the video, Ismail also explains that his oldest sister is also deaf, which gave him a reference in working with sign language during the episode: his character often acts as Cass' interpreter.

"I'm excited for my sister to see this," Ismail says.

Finally, both actors talk about signs they had to invent in sign language for certain words that appear during the episode. For example, they didn't have a word for the corporation that Cass and Lunn work for, Vector Petroleum, so they came up with a sign that represented the corporation's logo. There also is not a current word in sign language for "prototype," so the two actors created their own.

Don't miss the next episode of Doctor Who, "Under The Flood," which finishes the story started in "Under The Lake" on BBC America on Saturday, Oct. 3.

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