Both of the two main guest stars have been in the running to play The Doctor. David Warner was suggested by Pip Baker and Jane Baker for the Seventh Doctor prior to the casting of Sylvester McCoy. Liam Cunningham was seriously considered for the Eight Doctor in Doctor Who: The Movie (1996). Warner would later play an alternate universe version of The Doctor called The Unbound Doctor in multiple audio dramas for Big Finish Productions.
Steven Moffat had originally been hesitant to bring back the Ice Warriors, worrying that they were seen as "the default condition for what people thought of as rubbish Doctor Who (1963) monsters - things that moved very, very slowly and spoke in a way that meant you couldn't hear a word they said." Mark Gatiss, however, was a fan of the Ice Warriors' stories and had been campaigning to bring them back.
The things that are pulled out of The Doctor's pockets when they are searched are a Barbie, a ball of twine, a toffee apple and the sonic screwdriver.
It had been decided that the submarine would be realised as a model, rather than as computer animation, to provide a better sense of weight and presence.