- Awesome, Dear Boy: Eve Myles was initially not supposed to film the episode as she was booked for another role in theatre. However, her agent notified Myles of the role and Myles' was keen to audition for the series due to its reputation and Christopher Eccleston being "one of my favourite actors of all time".
- Cast the Runner-Up: Simon Callow was once a candidate to play the Eighth Doctor.
- Completely Different Title: The Hungarian title of this episode is "Testrablók" (Body Robbers).
- Development Gag: This is how Aliens in Cardiff came to be. Since BBC Wales films there, it's treated as a borderline Place Worse Than Death in-universe, with everyone astonished that anything interesting happens there. "I saw the fall of Troy! World War Five! Now I'm gonna die in a dungeon... in Cardiff."
- Irony as She Is Cast: Location variant. Despite the main studio for BBC Wales being in Cardiff, this episode, set in Cardiff, was actually filmed in Swansea.
- Never Work with Children or Animals: Small pieces of paper were sprayed as snow, which caused a problem as it scared the horses.
- Recast as a Regular: Eve Myles plays Gwyneth Cooper and then became a future relative of that character as Torchwood lead Gwen.
- What Could Have Been:
- According to Mark Gatiss, there was originally going to be a scene in which the Doctor was mistaken for Sneed's new cleaner. Someone would have stated, "I thought you'd be a woman" to which the Doctor replies "No, not yet", hinting that Time Lords can change sex. Would've become Hilarious in Hindsight when it turned out the 13th Doctor would be a woman.
- Gatiss' original script was bleaker and more frightening, including details about the previous death of Gwyneth's younger brother. He was advised by Russell T Davies to "make it more of a romp."
- At an early stage, under the title of "The Crippingwell Horror", the adventure was set at a "spiritualist hotel" owned by a Mrs Plumchute, and involved a psychic named Noah Sneed contacting the Gelth. The maid, Gwyneth, was a much more minor character at this stage; her brother, Davy, was interred at the nearby Crippingwell Cemetery.
- Gabriel Sneed was initially conceived to be a younger character, with David Tennant in mind for the role (which would have been Hilarious in Hindsight as well).
- Several characters were introduced and subsequently excised, including Mrs Sneed (who became superfluous as Gatiss strived to focus on Gwyneth) and a fake medium named Gideon Mortlock (who survived only in Gwyneth's mention of learning how to conduct a séance from a Madame Mortlock).
- During the scene in which the Doctor and Dickens are talking in the coach, the driver was supposed to shout down to them (referencing a Dickens work) and the coach was to crash, but this was too expensive.
- There was originally a scene where the Doctor responds to Rose's assertion that recorded history can't be altered by taking her to a devastated 2005, inspired by a similar scene in "Pyramids of Mars". Drawing the eerie depiction of George's travel through time in The Time Machine (1960), Gatiss scripted this to show the TARDIS scanner filling with Gelth-animated zombies as time marched on. However, it was ultimately felt that the return to the TARDIS interrupted the flow of the story too much and so the scene was removed, with the threat to the future now established through dialogue between the Doctor and Rose.
Trivia / Doctor Who S27 E3 "The Unquiet Dead"