The episode played on public expectations, as David Tennant's successor to The Doctor was soon to be announced. This was heightened by the fact that David Morrissey had apparently been in talks to play the Eleventh Doctor.
The original plan for the Christmas special was a fantastical adventure in which J.K. Rowling would appear as herself in a world driven by her own imagination. David Tennant disliked this notion, which he felt would veer too close to self-parody.
Russell T. Davies later realised that it would have been a better ending to have Miss Hartigan redeem herself by making the falling CyberKing disappear, rather than introducing what Davies calls "a silly Dalek continuum dimension vault" to the plot. Davies states that he "can't bear that there could have been a better ending than we actually transmitted".
This is the first episode to be broadcast after David Tennant's announcement that he would leave the role of the Doctor in 2010. However, it is the last episode to be filmed and produced before the announcement, as Tennant was playing Hamlet for the latter half of 2008 and early 2009
An early script draft had the story following on from the unused Cybermen cliffhanger for "Journey's End". The two inside the TARDIS would vanish, and the Doctor would realise that they were falling through the Time Vortex, and follow them to Victorian England.
Dervla Kirwan wore special black contact lenses for the scenes of the transformed Miss Hartigan. Because the lenses didn't completely cover her eyes, however, special effects team at The Mill had to electronically paint out any glimpses of white eyeballs that appeared in shot.
Russell T. Davies considered setting the story in the court of King Henry VIII, but rejected this notion on the grounds that it would not feature enough recognisible Christmas traditions (since these typically postdated the sixteenth century). He also mulled another completely new idea, about a hotel which becomes displaced in time.