"The Girl Who Waited" contains one of the most emotional uses of a multiple timeline storytelling device in all of science fiction. It not only tests Amy Pond's faith in her best friend — the Doctor — it reaffirms her love for her husband Rory.

But what it also does is show the affects of those emotional decisions on Rory and the Doctor too. As is so often the case, Rory steps up, part nurse, part soldier, while the Doctor engages with the bigger moral consequences of his decisions, both coming out of the experience visibly scarred in a way that the younger Amy—having not waited after all—does not.

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

(The episode is available on iTunes and Amazon.)

Having resurrected the Cybermen for the new Who era in "Rise of the Cybermen"/"The Age of Steel", writer Tom MacRae had worked on an idea for the Tenth Doctor called "Century House," in which the Doctor joins a live broadcast of the ghosthunting TV show Most Haunted (with genuinely paranormal results). Russell T Davies felt that the idea was too light, and close in tone to "The Unicorn and the Wasp" (the next script in that season) and shelved it in favor of his own "Midnight".

Tom's subsequent proposal for Steven Moffat was originally entitled "The Visitors' Room", and featured several supporting characters that didn't make it to the final script so that Old Amy could appear earlier in the story. Other working titles include "The Visiting Hour" and "Kindness," although a persistent fan rumor that the episode had ever been titles "The Green Anchor" has been vehemently denied by Tom MacRae.

https://www.youtube.com/watch?v=M7tsEpOi1y0

The cast list for this episode is the third smallest of any full length episode of Doctor Who, as five actors appear either on screen or in voice form. The First Doctor story "The Edge of Destruction" features four cast members—The Doctor, his granddaughter Susan and his companions Ian and Barbara—while "Heaven Sent" beats it with just three: The Doctor, Clara and the Veil.

In order to minimize Matt Smith's involvement with the story—so that he could be available to shoot "Closing Time" with James Corden—Tom came up with the idea of the Chen7 virus being fatal to the Doctor but not Amy or Rory. This meant Matt could shoot a good many of his scenes from the TARDIS set over a couple of days, rather than being on location.

The only other visible character in the story is the Check-In Girl, who mentions the "our roller-coaster zone, authentically modeled on the famous Warpspeed Death Ride at Disneyland, Clom." The planet Clom is, of course, the twin planet of Raxacoricofallapatorius, and the home planet of the Abzorbaloff, as featured in "Love and Monsters."

This story is not the first in which the Doctor has to accept the fatal results of a locked time stream. In "The Massacre of St Bartholomew's Eve," the First Doctor is forced to abandon Anne Chaplet to her death as meddling with her time stream would have catastrophic consequences. His companion Steven found this very hard to accept.

The Handbots were originally written as more druidic, robed creatures whose hands would appear from inside their robes. And during the development of their costume, their robot forms featured human heads as well as human hands. The split-skull “secondary delivery system” didn't emerge as an idea until shooting was due to begin.

The role of the older Amy Pond was intended to be taken by an older actress, to facilitate interplay between the two characters, but Karen Gillan lobbied hard to be allowed to play her older self, which required a lot of movement training and voice coaching:

https://www.youtube.com/watch?v=Q67k4Gcu1B8

There's a Harry Potter / Doctor Who crossover in this story, in that the voice of the Interface that helps Amy Pond to acclimate to her new environment is provided by Imelda Staunton, the exceptional British actor who is best known as Dolores Umbridge in the Harry Potter movies. As a sidenote, Imelda Staunton is one of the few Oscar-nominated actresses to appear in Doctor Who.

There's a superbly geeky tiny production moment as the Doctor is searching for his glasses in his toolbox. He activates a part of the TARDIS console that looks like a small reel-to-reel tape machine, and a reversed version of the original 1963 Doctor Who theme can be heard playing in the background.

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