Jonathan Groff had "never seen an episode" of 'Doctor Who' before landing his guest role.
Jonathan Groff needed a Doctor Who 'crash course'
The 39-year-old Broadway star - known for his roles in 'Hamilton' and 'Spring Awakening', as well as appearances in 'Glee' - is playing mysterious bounty hunter Rogue in this weekend's episode of the same name, and he needed a "quick crash course" in all this Whovian before starting work on the iconic sci-fi show.
He told the BBC: "I had known of 'Doctor Who' and how much of a big deal it was in the UK, but I had never seen an episode of it.
"But [showrunner Russell T. Davies] sent me about five episodes of 'Doctor Who' from throughout the years, including the first episode that ever aired in black and white.
"So, I had a very quick crash course before I started."
In 'Rogue', The Doctor (Ncuti Gatwa) and Ruby Sunday (Millie Gibson) arrive in 1813 to find guests at the Duchess of Pemberton's (Indira Varma) party being murdered.
Jonathan revealed he landed the role after getting a simple text from Russell, having admired the writer for a while now.
He said: "Oh my god! I got a very exciting text message from Russell T. Davies, and I'm a huge fan of his work, especially 'It’s A Sin' - I still haven't recovered from it!
"So I was very excited to hear from him. He just wrote to me and told me that there was this role that was available on the show and asked me to play it."
He admitted the "biggest challenge" was attempting to "understand and fit into the tone of the show", which meant finding a balance between "fantasy" and the "depth to the storytelling", which still feels "real".
The 'Frozen' star was helped by Ncuti and Millie, who were "big smiles and full of laughs", and made sure to give him a warm welcome.
He added: "I spent most of my time with Ncuti and I am obsessed with him. I think he is such a star.
"He's so unpredictable and thrilling to act with because you never see the same thing twice. He's so spontaneous, and yet so deeply connected to what he's doing. He really cares."