Clara and the Doctor have dimension trouble in "Flatline," with the Boneless traversing between two and three dimensions and the TARDIS becoming physically smaller on the outside.

And it's Clara's personality that takes on fresh dimensions as she not only lies about Danny's acceptance of her travels, but takes on the characteristics of her Time Lord best friend in order to become more Doctor-like, complete with sonic screwdriver and psychic paper.

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

(The episode is available on iTunes and Amazon.)

Jamie Mathieson, who wrote this episode, revealed that he had fewer pointers from Steven Moffat than he had for "Mummy on the Orient Express," but that one particular chilling moment came directly from the showrunner. He told Den of Geek: "A lot of the development of new ideas or directions would occur in a back and forth in meetings, so few things arrived in fully formed edicts as such. But I remember that the hand grabbing Al in 'Flatline' was totally him."

Bill makes reference to the "dead man's handle" on the train, which is a failsafe mechanism set up to prevent trains from rolling onwards — thereby causing accidents — if something has happened to the driver.

This isn't the first time the TARDIS exterior has been shrunk. In "Planet of Giants," the First Doctor, Susan, Ian and Barbara were all reduced to a fraction of their normal size, and the same thing happened again in "Carnival of Monsters," as the Third Doctor and Jo Grant materialize inside a miniscope. The Master interefered with the Doctor's TARDIS while he was trying to finally fix the chameleon circuit in "Logopolis," resulting in another shrinkage, while more recently the Eleventh Doctor materialized inside the Teselecta (in both "Let's Kill Hitler" and "The Wedding of River Song"), which required both the ship and Time Lord to take on a drastically smaller form.

There's some debate about exactly how big the TARDIS is on the inside, with various Doctors making different claims down the years, not least the Twelfth Doctor claiming to have "literally acres" of space inside, and that its full weight would "fracture the surface of the Earth." The Fifth Doctor estimated that jettisoning 25 percent of the TARDIS would provide "seventeen thousand tons of thrust" in "Castrovalva," Romana II claimed that the TARDIS interior would weigh "five times ten to the six kilos in your gravity" on the planet Alzarius, while in "Journey to the Centre of the TARDIS," the Eleventh Doctor claimed that the inside of the TARDIS was infinite, which is all the heavy there is.

British TV viewers will have been familiar with Matt Bardock, who plays Al, as he was completing a seven-year stint in the medical drama Casualty, playing paramedic Jeff Collier:

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

The Doctor has clearly revised his opinion again on whether he likes or dislikes learning things. Having told Clara in "Time Heist" that he "hates" not knowing what the Teller is, he's now back to the opinion shared by the War Doctor ("The Day of the Doctor") and the Tenth Doctor ("The Fires of Pompeii"): "Could you not just let me enjoy this moment of not knowing something? I mean, it happens so rarely."

There's an animator's production in-joke on the front of the first train, which has the number A113. This reference number came from the classroom A113, used by graphic design and animation students at the California Institute of the Arts, including film director Tim Burton, Pixar head John Lasseter and director/screenwriter Brad Bird. The number not only appears in their work, it also crops up in the work of other graduates, and has been picked up by other animators and graphic designers, and has become an Easter Egg. The number appears in most Pixar movies, TV animations such as The Simpsons, American Dad, South Park and Family Guy, and even live action films such as Spider-Man: Homecoming and The Hunger Games: Catching Fire.

Clara's instruction that the Doctor move the TARDIS "like... Addams Family" is based on the character Thing, created by American cartoonist Charles Addams. Thing is a bodyless hand that appears out of a small box in both the 1960s TV series and the 1990s movie franchise based on Addams's ghoulish cartoons.

When Clara claims to Fenton that she represents "Health and Safety" it's not a company or organization she's pretending to work for. The term refers to a particularly potent idea in British culture. Legislation to protect workers from unsafe conditions has been reported derisively in the press that "health and safety" became a catch-all term for interfering do-gooders with a less-than-firm grip on common sense. That's why the Tenth Doctor and Donna Noble claimed to be from Health and Safety when gaining entry to Adipose Enterprizes ("Partners in Crime") and why the Eleventh Doctor said the same thing to get into Caliburn House ("Hide").

Christopher Fairbank, who plays Fenton, is another familiar face for British TV viewers, but it's his voice that Whovians will have experienced before. In the Big Finish audio story "The Wreck of the Titan," he played Pierre Aronnax, and he was Doc Baroque in "The Scapegoat" and Marc Brunel in "Iron Bright."

But he's best known for the role of Liverpudlian plasterer and former convict Moxey in the comedy drama Auf Wiedersehen, Pet, written by British comedy legends Dick Clement and Ian La Frenais.

https://www.youtube.com/watch?v=4hFBIlIx0jY

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