"Night Terrors" was created as a result of writer Mark Gatiss immersing himself in his first love, horror movies. It's a thoughtful update of several key horror themes - haunted houses, the creepiness of innocent toys, aliens living among humans - with a few personal phobic touches. And in the eerie metamorphosis from human to peg doll, one of the uncanniest transformations in Doctor Who's long and magnificent history of uncanny transformations.

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

(The episode is available on iTunes and Amazon.)

Mark Gatiss came to his third Doctor Who script (after "The Idiot's Lantern" and "Victory of the Daleks") with a concept inspired by his time working on a three-part horror series for the BBC called Crooked House. One of his ideas had been to have a modern home be possessed in the vein of the classic haunted house. Steven Moffat liked this idea, and asked Mark to adapt it for Doctor Who. Potential locations included a hotel hosting a psychiatric conference, in which the phobias of the attendees would influence the ghostly apparitions they had to face. Ultimately this was felt to be too similar an idea to Toby Whithouse's "The God Complex," so Mark concentrated more on irrational childhood fears.

He used three items from his personal list of phobias in the story; the peg dolls, based on a lifelong aversion to dolls in general; a horror of piles of garbage in bags, and an aggressive gym teacher from his youth, whose domineering persona went into the landlord Purcell.

The peg dolls in George's doll's house are based on a type of wooden doll that originated in the Val Gardena in the Alps, and were common to Germany and the Netherlands. These were created using lathe-turned parts, held together with wooden pegs, hence the name peg dolls, or Dutch dolls. The dolls were sold without clothes, with the idea that children could make their outfits from scraps of leftover material, adding to their slightly homemade appearance.

Mrs Rossiter says she can go "up and down them stairs like Sherpa Tenzing," a reference to Tenzing Norgay, the Nepali mountaineer who was one of the first two people to have reached the summit of Mount Everest, with Edmund Hillary on 29 May 1953.

As this story was originally intended to form part of the first block of Season Six, some elements of the continuing story had to be adjusted, given that they formed part of the runup to "A Good Man Goes to War." A scene with Madame Kovarian was taken out, and the Doctor's line "it's good to be all back together again, in the flesh" had a double meaning, given that Amy would have been Ganger Amy at that point in the storyline.

The Doctor's list of childhood nursery stories includes "The Three Little Sontarans," "The Emporor Dalek's New Clothes" and "Snow White and the Seven Keys to Doomsday." This last one is a nod to a retelling of the story of Snow White that appeared in the 2012 Doctor Who Christmas annual. The plan had been that the story would be referenced in "Night Terrors", then you could read it in the annual. However, "Night Terrors" was moved to the latter part of Season 6, which meant the story came out first.

The title of that story is a further nod to the 1974 stage play Doctor Who and the Daleks in the Seven Keys to Doomsday, written by Doctor Who scriptwriter Terrance Dicks and starring Trevor Martin as the Fourth Doctor (the play started its run two weeks before Tom Baker was confirmed as the Fourth). Also among the cast was former Second Doctor companion Wendy Padbury, playing a different companion called Jenny. The story—parts of which were recycled by Terrance in the Fourth Doctor TV adventure "The Brain of Morbius"—was eventually recorded for a 2008 Big Finish audio production.

Mark Gatiss put the words "maybe later" into the script (Alex says it to Purcell the landlord) as a nod to a shared joke he had with Daniel Mays when the two worked on a BBC comedy thriller called Funland. But it was possibly too subtle, as Mark ruefully told Radio Times: "'Maybe later'" became a bit of a catchphrase for me and Danny when we worked together on Funland, so I added it as a line in Night Terrors. I thought he’d be tickled – he didn't remember it at all!"

While Mark Gatiss's genuine fear of dolls had fuelled this story, it's not the first time the Doctor has faced a transformation into toy form. In 2010, the Eighth Doctor was changed into a doll by his old foe the Celestial Toymaker in the Big Finish audio adventure "Solitaire".

Mark Gatiss's original titles for the story were "House Call" and "What Are Little Boys Made Of," the latter taken from the early 19th century English nursery rhyme, which is often attributed to the poet Robert Southey:

What are little boys made of?

What are little boys made of?

Snips and snails and puppy-dogs' tails

That's what little boys are made of

What are little girls made of?

What are little girls made of?

Sugar and spice and everything nice

That's what little girls are made of

The "snips" in the boy verse (a Cumbrian slang term for a small eel) is often swapped out for other disgusting animals: slugs, frogs or snakes.

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