So the way it worked was this: Steven Moffat gave me the title: 'Mummy On The Orient Express' and the idea that this episode would be set in space. I went away and had a think about it. And what I thought was this:
A monster on a train in space is a problem. Put bluntly: there's nowhere to hide it. As soon as someone is found dead by monster, people will search the train. So where is it hiding? Is it someone's alter ego, like Jekyll and Hyde? Is it fading in and out of visibility? Is it a ghost? Does it assemble and disassemble into snake like bandages? Do we set up arbitrary locking points along the train? Does it walk outside the train in the cold of space?
All the possible solutions I thought of felt a little meh, or made the passengers and the Doctor seem dumb for not finding the mummy's hiding place in five seconds flat.
While tinkering with all the various permutations of visibility and oblivious passengers, I started thinking about what scares you as a child. Monsters, obviously. Under the bed, in the closet, in the shadows. But also the idea that when you run and tell your parents about the monster, they tell you that there's nothing to be scared of. The monster isn't real. You were imagining things. But you know you weren't. And your parents denial of your monster makes it even scarier. You are the only one who can see it. You must face it alone.
That idea was in place in the first two page rambling I submitted on the episode. The monster that can only be seen by the intended victim. Other elements of the Foretold mythos came and went. Some of them I hesitate to mention as they may find their way into the DNA of future monsters (remember kids, use every part of the buffalo) but certain off cuts are fun to disclose.
There was a beat where the Doctor figured out how to reveal the Foretold, pulled a switch... and twenty Foretold faded in.
There was the realisation that the only way to beat the Foretold was to crash the train into a planet full of things worse than them. Which kind of weakened the Foretold's scary factor a little.
There was Clara seeing the Foretold, and hiding inside the sarcophagus, which was then revealed as actually being a Foretold making machine, wrapping her in bandages...
All fell away over time, simplifying the narrative. The set piece of Quell's death was a favourite and we realised that the timed deaths should be the crown jewels of the episode, so more were added, of course culminating with The Doctor finally seeing and beating the Foretold.
The mummy to me has always seemed a bit of a poor cousin to the much cooler vampire, werewolf and zombie, but if we've done our job well this episode may go some way to redress that.
I hope you enjoyed it.