"A Town Called Mercy" was the first trip the TARDIS took back to the old west since the First Doctor paid a visit to the OK Corral in "The Gunfighters" in 1966. But as Steven Moffatt wanted the first half of Season 7 to be a series of stand-alone adventures with their own distinct identities, a trip to Texas seemed a great idea.

The brief he gave to Toby Whithouse was that a western town was being held under siege by a giant robot, what Toby wrote was far more emotionally complex, dwelling on brutality and forgiveness, and the ever-present chance of redemption available to anyone willing to take it.

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

(The episode is available on iTunes and Amazon.)

Filming for this episode took place in the Tabernas Desert region of Almería, Spain, on the same sets Sergio Leone used for his "spaghetti" westerns such as A Fistful of Dollars. These had been maintained as cowboy theme parks, one called Oasys (or “Mini Hollywood”) and another called Fort Bravo (or “Texas Hollywood”).

https://www.youtube.com/watch?v=Syqm8v3Xr1g

The script was originally written under the working title of "The Gunslinger," which was changed first to "Mercy" to avoid confusion with the Stephen King novel of the same name, and then "A Town Called Mercy," to make it sound closer to westerns such as A Man Called Horse.

When drawing inspiration for his script, Toby Whithouse drew on the iconic tropes of the Leone westerns, but with the modern emotional complexity of HBO's Deadwood. He deliberately didn't watch the First Doctor story "The Gunfighters," because he had been told it was "not exactly the jewel in the crown" of Doctor Who stories, and wanted to avoid any unconscious influence.

https://www.youtube.com/watch?v=hfaSsaHUe34

Toby had originally planned that Kahler-Jex would be shot and killed by Isaac's father George mid-way through the story, with the avenging Kahler-Tek looking to destroy the town now his revenge could not be complete. The Doctor was to defuse the situation by showing Kahler-Tek a projection of a woman wearing the same pendant as him, to remind him to protect innocent lives.

Another plot twist that didn't make the final cut was that Kahler-Tek would try and spook his prey by playing a snatch of beautiful music just before he arrived. This would later be revealed to be the music Kahler-Jex played while operating on his various medical subjects to drown out their screams, but as a narrative device, it's a nod to the pocket watch in this scene from Sergio Leone's For A Few Dollars More:

https://www.youtube.com/watch?v=xpkA68e5HN4

Ben Browder, who plays Sheriff Isaac, is rightly famous in sci-fi circles for his roles in Farscape and Stargate SG-1, but his appreciation of Doctor Who comes not from immersion in fan culture, but from having studied at the Central School of Speech and Drama in London. He is married to British actress Francesca Buller, and was not only well aware of Doctor Who, but also of the show's central place in British popular culture.

Garrick Hagon, who plays the undertaker Abraham, has previous history with Doctor Who, having appeared as Ky in the Third Doctor story "The Mutants," and The Jester in Big Finish's audio drama "The Axis of Insanity." But he's best known to sci-fi fans as Luke Skywalker's best friend Biggs, in Star Wars.

When discussing his apparent maturity, the Doctor says, "I'm twelve hundred years old now. Plus I don't want to miss The Archers," a reference to the world's longest running radio drama, on BBC Radio 4. A British instition, The Archers has been on air since 1950, depicting events and dispensing invaluable farming advice in the fictional rural village of Ambridge. It also has one of the most distinctive musical themes in British broadcasting history—as iconic as the Doctor Who theme—the ever chirpy "Barwick Green"

https://www.youtube.com/watch?v=wNEawM6vZf4

As is often the case with Doctor Who, there's scope to imagine where in the timeline of events this standalone adventure takes place. An intriguing clue came from the part-work Doctor Who magazine The Complete History, which states that its in the original production notes that the trip to Henry VIII's court that is mentioned here (and Rory leaving his phone charger behind) is the same trip we see in "The Power of Three," which is of course the next adventure in Season 7.

While inside Kahler-Jex's ship, the Doctor says, "This is an awful lot of security for a titchy spacecraft," using a fine British slang term that means small and cute, in an unimpressive sort of way. The term came into popular use via two interesting routes. The hugely music hall entertainer Little Tich was four-and-a-half feet tall, and almost as a mark of affection, the public came to refer to little things as "tichy" or "titchy" to denote meagre stature. But there's another twist. Little Tich (real name Harry Relph) only acquired his stage name because of a facial resemblance to Arthur Orton, an Australian butcher who had come forward in the late 1800s claiming to be Roger Tichborne, the long lost heir to the Tichborne baronetcy who was believed to have died in a shipwreck in 1854. It was such a high profile case that Relph could trade on that likeness in the early stages of his career.

https://www.youtube.com/watch?v=a5ZXb61-BQw

Now go back and read the entire 10 Things You May Not Know About Doctor Who archive.