Inspirations: Knock Knock

A fairly straightforward one this, as it's another attempt to do a "haunted house" story in the series. There had obviously been an element of this previously in Blink, and more so in Hide. 

Specifically, what inspired writer Mike Bartlett were those odd creaks which people hear in the middle of the night, or when you are supposedly all on your own in an empty building... 

Promoting the series Doctor Foster, Bartlett had mentioned that he was a Doctor Who fan, and this was noted by the series' script editor - Lindsey Alford - who then mentioned it to her husband, who just happened to be its executive producer Brian Minchin.

Steven Moffat assumed that the writer would be far too busy to submit anything as his own series had a second season commissioned, but Bartlett insisted he would make the time.

Sent away to think of an idea, he hit upon creaking floorboards, and pondered if these were caused by something within the very fabric of a house. There would be something hidden in the attic, and the walls would move.

Always a fan of exploiting a common fear for the show, Moffat was pleased with the concept.

Thoughts about the housing crisis led him to plot a story about a group of people forced to move in and share - only to be picked off one by one by whatever lurked within the house. When given details about new companion Bill, who was to be a university student, it made sense to have the sharers a group of her student friends moving in together.

Bartlett had once lived in a big Victorian house in Yorkshire as a student, which had a basement that looked like a dungeon.

The menace in the walls would be giant insects, and Bartlett named these Dryads - after the wood nymphs of Greek mythology.

"Drys" is the Greek word for oak, and it was initially believed they were specifically spirits of those trees. Tied to a particular tree, they died when it died, so they weren't immortal. Their appearance changed with the seasons.

Two notable Dryads were Daphne - who was transformed into a laurel tree to avoid the unwanted attentions of Apollo - and Eurydice, wife of Orpheus, whom he tried to bring back from the Underworld.

Bartlett envisioned actors Mark Rylance or Stephen Rae for the Landlord - a mysterious man in a brown suit. In the end, he would be gifted David Suchet, famous for his long-running TV role as Poirot, which ran from 1989 to 2013. (Peter Capaldi had featured in the third season story Wasp's Nest).

Suchet accepted the part without even asking what it was, so pleased was he to appear in the series.

The look came from a man who used to visit his parents, but the brown suit fits with the general colour palette of the episode. It was intended that the house would have wooden doors and wood-panelled walls.

The thing in the attic would prove to be a wooden woman - inspired by The Sandalwood Girl. This 1985 children's novel was written by Sheila K McCullagh, and Bartlett had been reading it recently to his son.

In the first draft, the house would give up all its victims at the end - going back every 30 years, to 1937. The Doctor would offer to get them all back to their own times in the TARDIS. The first disappearances, according to the second draft, were in 1927.

Until late in the day, the house was to have been restored at the end of the episode, but this was changed in order to explain why Bill would be back staying with her foster mother again later.

One of Bill's new flatmates is named Harry, and it was originally intended that he would be the grandson of former companion Harry Sullivan. It was decided that this was too obscure a reference and was dropped - despite Harry being referred to fairly prominently in Inversion of the Zygons only the year before...

Harry was then going to become an ancestor of one of the colonists from the earlier episode Smile.

At one point the Doctor talks about "going up the wooden hill" - meaning up the stairs to bed. (The full saying is "up the wooden hill to Bedfordshire"). He had previously used this phrase to Ace in Ghost Light, which Bartlett considered another inspiration.

The building is described by student Shireen as a "Scooby Doo house", as in the spooky mid-1970's cartoon series featuring a Great Dane and a bunch of teenage mystery solvers. 

Larry Nightingale had described Wester Drumlins in the same way in Blink - and the same location was used both in that story and in this, though a different section of the building.

David Suchet would be surprised to discover that his family had rented the property for a holiday a couple of years before.

Telling the students about the time he helped out musician Quincy Jones, the Doctor mentions that he had a Klarj Neon Death Voc-Bot for a bassist - presumably a reference to the ones seen in The Robots of Death.

Next time: The Suits get you every time. Bill encounters zombies - in space. The Doctor turns a blind eye...