Michael Troughton, son of Patrick Troughton and brother of David Troughton, makes his first appearance in the series.
This was meant to be Clara's last story. Jenna Coleman had originally planned to leave at the end of season eight, but she enjoyed working with Peter Capaldi and decided to stay for the Christmas special. As such, the first draft ended with the Doctor bidding farewell to the aged Clara. There was then a dream sequence in which Clara imagined herself ascending the spiral staircase which had led to the TARDIS in The Snowmen (2012). As she climbed the steps, her form shifted between the various echoes of Clara which had been seen during season seven. She finally became the younger Clara who had travelled with the Doctor, and smiled as the TARDIS flew past on its way to the stars. This coda adapted an idea which Moffat had originally conceived for the end of The Day of the Doctor (2013). At another stage, she was supposed to die of old age. Capaldi convinced Coleman to stay on for season nine, prompting Steven Moffat to rewrite the ending.
References are explicitly made to Alien (1979), which was of course a significant film in the career of John Hurt, who would later go on the play the Time War incarnation of the Doctor. The film was directed by Ridley Scott. In 1963, Scott was a trainee set designer, who was assigned to design the Daleks. However, Scott had to back down, because of a schedule conflict, and was replaced by Raymond Cusick.
This is the third consecutive Christmas special to feature Dan Starkey. He had previously appeared in his recurring Sontaran role, Strax, in The Snowmen (2012), then as both Commander Skarr and his unnamed subordinate in The Time of the Doctor (2013).
When Clara and the Doctor leave the base for the first time, as soon as they exit and go outside, Peter Capaldi sneezes, but the actors continue on. This take ended up in the final episode rather than another one, but the Doctor's sneeze is left silent.