This episode adds an electric guitar counter-melody to the title theme, played by Peter Capaldi himself, segueing from the Doctor playing Beethoven's Fifth Symphony on his electric guitar at the end of the cold opening. This is a one-off variant that is not repeated in the succeeding episodes, though it has become popular with fans, who enjoyed it.
The Doctor's guitar amplifier is branded Magpie Electronics - a nod The Idiot's Lantern (2006) when The Tenth Doctor and Rose go back to the Queen's coronation. The maker of the possessed TV sets was Magpie Electronics.
Steven Moffat believed that two-part stories worked best when the second installment diverged in some significant way from the opening episode. As such, Toby Whithouse decided that the storyline would open up beyond the underwater base. Having long been intrigued by remote military outposts, he chose a deserted army installation as the new setting.
A late change was to the fourth-wall-breaking explanation of a bootstrap paradox, a scene which Toby Whithouse was surprised to be allowed to retain at all. Originally, the Doctor told a story about a time traveller who accidentally killed Leonardo da Vinci in infancy, and used documents he had brought with him from the future to forge all of Leonardo's discoveries and accomplishments. Later, this became the Doctor's possession of a postcard of The Last Supper, which in turn became the basis of Leonardo's genuine masterpiece. However, it was then decided to tie the scene into The God Complex (2011)'s introduction of the Doctor's electric guitar. The subject of the monologue consequently changed from Leonardo to Ludwig van Beethoven.
The funeral director's calling cards are in the shape of a gravestone, and read, 'Albar Prentis, Universal Funeral Director - May the remorse be with you'