Moffat said that the Doctor will meet the Mother Superious of the Papal Mainframe, calling her an old friend from his distant past.
“[The makeup] was a [Bladerunner] reference, actually,” Brady said. Moffat asked the makeup artist for a look that conveyed the idea of “slightly not-quite-human, three-quarters human thing.”
Brady also mentioned having a hard time not breaking character with laughter as she heard the Doctor’s interactions with other characters:
“Not my character. It was Matt’s interaction with a certain other character who I’m trying not to name [laughing]. There were other conversation strands between the Doctor and somebody else that made me laugh out loud. There were a couple of moments between Tasha and the Doctor that made me laugh out loud, but she’s not a particularly funny character …”
Brady also said that the special was “firmly about wrapping up the Doctor’s story.”