- Ascended Fanon: The notion that Nardole has invisible hair has been circulating as a Running Gag among fan shows and podcasts for a while now. (No one's claiming credit for the glass nipples, though.)
- Descended Creator: Two of the show's writers take roles in the episode, Mark Gatiss playing the Captainnote and Toby Whithouse playing the German soldier. Steven Moffat comments on the meta aspect involved in a Doctor Who Magazine interview:"I just love that in my very last episode, there's my writers' room! There's the two stars of my writers' room, with guns pointing at each other, in a crater!"
Steven chuckles at the thought. What symbolism is going on there?
"Who can say? Who wants to unpack that one?"
- I Knew It!: Most fans had figured out that the Captain was somehow related to the Brigadier long before the special aired.
- A minor one. In the Expanded Universe, the First Doctor is often depicted using a sonic screwdriver of the same model as the Second. This episode shows thats not possible: In this episode set during the First Doctors final story, he doesnt recognize the sonic screwdriver, and moreover, isnt impressed in the slighest with "that ridiculous buzzing toy". As such, not only has One definitely never had one, Two probably didnt have one offscreen for a long time before we saw it.
- Some fans had theorized that the Captain was in fact Gilbert Mackenzie-Trench, the man responsible for the iconic police box design. This was due to the fact that he had fought in World War I and bore a passing resemblance to Mark Gatiss.
- A very popular theory about the plot was that it would turn out to be the First and Twelfth Doctor's sides of "The Day of the Doctor", finally explaining when and where Twelve was when he saved Gallifrey, partially because it would have made ideal Bookends to his onscreen debut in that special, partially because multi-Doctor specials usually have extremely high stakes. Instead, Twelve's participation remains a Riddle for the Ages.
- There was a lot of hope that Susan Foreman would appear, finally providing resolution to the question of her fate in the Last Great Time War, owing first to her being referenced in "The Pilot" (which previously made her a surprisingly popular candidate for the Vault's occupant until "Extremis" Jossed that) and second to the reveal that the First Doctor was appearing in this story. It helped that Peter Capaldi had hoped his Doctor would meet her, and the other characters he'd hoped Twelve would meet at some point the Mondasian Cybermen had appeared in the first stretch of this Grand Finale storyline precisely because of his desire. More than a few viewers expected her to be the person the Testimony presented to the Doctors and then Glass!Bill's voice was heard. (The Novelization notes that 1) Testimony only collects human memories, 2) the First Doctor himself initially thinks the woman is Susan, and 3) at some point he DID see her and her family prior to the events of "The Tenth Planet", though he'd hoped to see her one more time before his regeneration/death.)
- Some fans believed that the Doctor clearly seeing Clara Oswald in his mind at the end of "The Doctor Falls" meant he'd regained his memories of her due to his near-regeneration experience, but the denouement of this story involves their formal restoration via the Testimony. The Novelization fills this in he recognized Clara, but deliberately forgot it as soon as he did because he always remembered why he had Laser-Guided Amnesia for her sake.
- In a truly unusual case of this trope, most fans expected Bill Potts to appear upon the announcement that Pearl Mackie would appear in this episode. Technically she doesn't, and a resultant running question of the episode is whether Glass!Bill can be counted as the Bill. Some fans did correctly predict something was up with "Bill" because 1) Heather wasn't confirmed as appearing too, 2) Mackie hinted in interviews that there would be a twist to her appearance, and 3) one piece of promotional art actually hints at her true nature by having her feet look suspiciously glassy! (The Novelization does give full detail on Bill and Heather's Surprisingly Happy Ending.)
- Mythology Gag: The Testimony - a benevolent intelligence which extracts humans (and specifically humans) from the timeline shortly before their deaths and collects their memories to recreate them in an Artificial Afterlife - is strikingly similar to The City of the Saved.
- Name's the Same: Shares a title with an animated film, whose plot coincidentally also involves time being frozen thanks to the heroes!
- The Other Darrin:
- Barring stunt doubles and a couple of instances where we never see their face, David Bradley is only the second person ever to play a version of the Doctor whom we've already seen played by a different actor. Oddly enough, the first time around also involved the First Doctor reappearing for a multi-Doctor special. The original First Doctor, William Hartnell, was replaced by Richard Hurndall for "The Five Doctors" due to Hartnell's Author Existence Failure. Hurndall has since also become a victim of Author Existence Failure, hence Bradley's casting to replace them both. That's what happens when a Long Runner has a character who needs to look old and white-haired.
- Polly and Ben are played by new actors since Polly's actor has long since aged out of the role and Ben's actor passed away very suddenly decades before this story aired (having the sad distinction of being the second actor who played a companion to leave us, after Ian Marter). Unlike the First Doctor, Polly is played by a different actress (Lily Travers) to the one who played her original actress, Anneke Wills (Ellie Spicer) in An Adventure in Space and Time.
- Reality Subtext: Like the last two Doctors, Twelve's final words heavily reflect the circumstances around the actor change. Except this time he speaks not just for the actor but also for Steven Moffat, giving his advice to new showrunner Chris Chibnall.
- Recycled Script: A Steven Moffat story in which a giant computer that uploads people's memories at the time of their death plays a big role. Haven't we heard that one before? There's a reason the Testimony is described as "Heaven on New Earth"!
- Role Reprise: Meta-example. David Bradley had previously played The First Doctor by proxy of playing William Hartnell himself in An Adventure in Space and Time. This special cuts out the middleman and has Bradley playing The Doctor directly.
- Throw It In!: Moffat hastily added the bit about children hearing the Doctor's name to Twelve's final words after Capaldi came up with it during a convention panel.
- What Could Have Been:
- This special could have been the Thirteenth Doctor's first adventure... or might not have existed at all. Steven Moffat and Peter Capaldi intended "World Enough and Time"/"The Doctor Falls" to be the Twelfth Doctor's Grand Finale, leaving the 2017 Christmas Episode to incoming producer Chris Chibnall and Thirteenth Doctor Jodie Whittaker. But Chibnall didn't want that, and when Moffat learned that Who wouldn't have any more Christmas specials if the 2017 show were skipped (the BBC being busy enough at Christmastime with its schedule as is), he convinced Capaldi to come back for one more episode and revised the script of "The Doctor Falls" to end with the Doctor postponing his regeneration, leading into this adventure. Despite Moffat's efforts, however, this ended up as the last Christmas Episode after all, as Chibnall subsequently decided to move the annual post-season special to New Year's Day.
- Bill Potts was not included in early drafts of the script, with Steven Moffat intending The Captain to be The Watson, but he changed his mind when he realized that there needed to be more "fun" in the story.
- Moffat also planned to finally explain why the First Doctor's clothes changed to fit the Second Doctor upon regeneration, but decided against it.
- Production set photos released before the episode revealed more recreations of sets from "The Tenth Planet" having been built, as well as Mondasian Cybermen being prepped to appear, all of which imply that the opening flashbacks to said serial were going to be more involved than in the final episode.
- Indeed, a more extensive behind-the-scenes documentary aired after the episode in North American movie theaters shows footage of much more extensive recreations, what might have added up to several minutes of scenes from "The Tenth Planet". One of the creators even commented that it shows how they could potentially recreate missing episodes wishful thinking, or the BBC putting out feelers on potential audience reactions?
- Further confirmed by the May 2018 reveal that the original cut of this special was 90 minutes. That's a half-hour's worth of Deleted Scenes!
- As originally written, Clara's avatar appeared alongside those of Bill and Nardole, with the Doctor's group hug taking them all in. Moreover, rather than the "empty battlefield" speech serving as the Doctor bidding the avatars adieu, he instead would have teased them by saying that one of them was his favorite companion but not saying who, to the ladies' confusion (while Nardole assumes it's him), which would have given his return to the TARDIS a much lighter tone. However, Jenna Coleman could not afford to come to the battlefield location due to her commitments to Victoria, so Moffat came up with a bit that could be quickly shot in a studio without any other actors involved, which is why Glass!Clara is alone in all shots of her in the finished episode.
- Word of God: Mark Gatiss specified that the Captain is the Brig's grandfather. The people in charge of the Brig's intellectual property rights disagreed and said he was actually the Brig's great uncle (since a different character had already been established as the grandfather in other media). This dispute was resolved with the release of a short story that implied the Captain was the biological father of the Brig's dad as the result of an affair.
- You Look Familiar:
- David Bradley is now playing the First Doctor, a large contrast to the villainous Solomon. And for those who have seen An Adventure in Space and Time, he's playing the exact same role, only this time he's the actual character, not the actor who played the character.
- Mark Gatiss shows up again, in his fourth acting role in the series after previously playing Lazarus, Gantok, and a Spitfire Pilot, and the third of which he is credited for (he was credited as "Rondo Haxton" for Gantok and went uncredited as the Spitfire Pilot due to providing just his voice as a very minor character insert comparable to an Easter Egg).
Trivia / Doctor Who 2017CS "Twice Upon a Time"