Although the action was normal in the classic series, this is the first time since the series restarted that the Doctor has closed the TARDIS doors from the console, rather than manually shutting them or occasionally closing them by snapping his fingers.
Steven Moffat gave the concept of an episode discovering the centre of the TARDIS to Stephen Thompson. Thompson explained that this was because Moffat was "haunted" by Doctor Who: The Invasion of Time: Part Six (1978), which was set on the TARDIS but used hastily-constructed sets. Thompson was also interested in mathematics and remarked, "anything involving multi-dimensional geometry gets me excited".
Steve Thompson initially thought back to his days as a teacher, and came up with a story in which the TARDIS crashed into a school trip, unleashing a gaggle of teenagers into the time machine who cause it to malfunction. Steven Moffat disliked this idea, and so Thompson suggested replacing the students with a salvage team.
Ashley Walters managed to get in trouble with the producers on the first day of filming when he tweeted a picture of himself in his costume in his trailer with the word "space". The picture was immediately removed.
In early versions, Clara came across a chamber containing the leftover belongings of all of the Doctor's previous companions; this was eventually simplified to the storeroom in which she finds one of the young Amy Pond's model TARDISes (from Doctor Who: Let's Kill Hitler (2011)) and the Doctor's cot (from Doctor Who: A Good Man Goes to War (2011)).
While Steve Thompson was writing the episode, it was intended to be the sixth episode of the spring run. As such, it ended with a coda featuring Clara's charges, Angie and Artie Maitland, confronting the time travellers about their adventures and leading into episode seven, Doctor Who: Nightmare in Silver (2013). Subsequently, Thompson's story was moved to the fifth spot, and the epilogue was rewritten and shifted to the eventual episode six, Doctor Who: The Crimson Horror (2013).
Nearly all of the episode was filmed at Roath Lock studio home. An exception to this was the first day of recording, September 4th, when the deck of the Hornet was actually a warehouse on Celtic Way in Newport.
This is the first episode since "The Web Planet" to use the "British" spelling of a word "Centre" versus "Center", although other episodes don't use words that have different spellings in Britain and America.
In the final conversation between Clara and the Doctor, you can tell there are two distinct takes making up the scene because Clara's hair consistently shifts between being on either side of her head when the camera is facing her and being all pulled over her left shoulder and even off her neck when the camera's behind her.
The trivia items below may give away important plot points.
Originally, the third van Baalen brother was named Sander rather than Tricky - meant to be short for "Electricky" - and was not an android, while Gregor was disfigured and had numerous metal attachments.
While in the Library, Clara comes across a book titled The History Of The Time War. She then opens the book to an undetermined page and says "So that's who." Leading one to think she knows who the Doctor is.