"The Next Doctor" is one of the most beloved adventures in Doctor Who's long history. It's partly because the premise is adventurous and mysterious. Without getting too far into spoilers, the Doctor meets a man called the Doctor, with a companion and sonic screwdriver and a TARDIS, in Victorian London. Oh, and there are Cybermen.

It is also partly because of David Morrissey's wonderful, emotional performance, which has left fans quietly wondering if he couldn't one day be in line to take on the role of a certain gallivanting Time Lord for good.

Here are a few things to keep an eye out for, the next time you watch:

(The episode is available on Amazon and iTunes.)

One of the alternative storylines Russell T Davies considered for this Christmas special involved J.K.Rowling, who would appear as herself, stuck in a fantastical reality which was dominated by the figments of her own imagination. Although tempting to cross fandoms in this way, it was felt to be a step too far out of the 'reality' of Doctor Who's universe. One of the people who was particularly against the idea was David Tennant.

Russell also looked at previous Christmas stories as potential inspiration, particularly Hans Christen Andersen's "Little Match Girl" (which appears to have largely influenced his development of the character Rosita). Trying to avoid setting another Christmas special in Victorian London — after "The Unquiet Dead" — he considered setting his story in the court of Henry VIII, but couldn't work in the Christmassy feeling he wanted, as most of the recognizable British festive traditions came after his reign.

Having worked out that the story would involve a man who claimed to be the Doctor, Russell T Davies wanted to title the story "The Two Doctors," but this is already the title of a Sixth Doctor adventure in which he runs into his second incarnation. Russell then opted for "The Next Doctor," then "Court of the CyberKing" and then back to "The Next Doctor" again.

https://www.youtube.com/watch?v=Bz96RhZ4Vlg

Since the episode aired, Russell T Davies has said he'd love to write a Doctor Who novel which takes place in the brief moments between Jackson Lake entering and leaving the TARDIS. There's room in there for an adventure or two, with the Doctor returning Jackson to that exact spot, just in time to go for Christmas dinner.

There are a couple of notable literary references in the script. When he first arrives, the Doctor asks a boy, "What year is this?" and receives the reply: "Year of our lord 1851, sir," which is a nod to a similar question and answer in Charles Dickens' festive favorite A Christmas Carol. Then Jackson Lake calls the Cybershade a "timorous beastie," echoing a similar comment made by the Doctor about Rose Tyler, when he met Queen Victoria in "Tooth and Claw." In both cases, the quote comes from the 1785 Robert Burns poem "To A Mouse," the first line of which is: "Wee, sleekit, cow'rin, timorous beastie."

David Morrissey told the Radio Times that he based his performance as Jackson Lake acting like the Doctor on three particular actors: "When I look at Tom Baker and William Hartnell, there's a truth to their performances; Patrick Troughton as well. They never saw [Doctor Who] as a genre show or a children's show."

https://www.youtube.com/watch?v=6gKi3BM1MN8

The full backstory for Miss Hartigan was a lot more disturbing than could be shown on the TV. In a BBC 7 radio documentary called Doctor Who: The Commentaries, Russell T Davies describes her as "a victim of abuse," but stopped short of saying exactly what her social standing was, apart from noting the importance of her scarlet dress: "I'm talking quite discreetly around this because there are children listening and watching and there's only so far I should go." He also added that she was "a powerless woman who's been in servitude or far worse all her life," and in order to transcend her past, she effectively "becomes a man, she becomes the CyberKing. She has to go through this extraordinary process because she's so damaged."

One element for which Russell voiced his dissatisfaction was the grand finale. The convenience of using the Dalek dimension vault to banish the CyberKing to the void nagged at him, especially once he realized there was a potentially better way to end the story. In Doctor Who: The Commentaries, he said that Miss Hartigan "should have destroyed the Cybermen when she screamed" but goes down with the CyberKing, leaving the Doctor pleading with her to save the city below. Then she could trigger the dimension vault herself, as a final act of humanity, and redeem herself. Davies went on to say he "can't bear that there could have been a better ending than we actually transmitted."

When putting together his ideas for the Cybermen we see in his Eleventh Doctor adventure "Nightmare in Silver," Neil Gaiman revealed that the Doctor banishing this particular Cyber cohort into the void would go on to have an unexpected effect, involving a cross-breeding between those which were originally built by Cybus Industries and the classic era Doctor Who Cybermen from Mondas and Telos: "My theory is the Cybus Cybermen were sent to Victorian days and zapped off into time and space at the end of 'The Next Doctor.' They met a bunch of the Mondasian/Telosian Cybermen, and there was some cross-breeding and interchange of technology, which is why you then get the ones that look like, but actually aren’t, the Cybus Cybermen."

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