Even the Doctor Who Christmas special director doesn’t know who will replace Peter Capaldi

Rachel Talalay, the director of Peter Capaldi’s final episode, claimed on Twitter that she doesn’t know who will be the Thirteenth Doctor. Talalay said that she had "no knowledge" of which actor will replace Peter Capaldi.

https://twitter.com/rtalalay/status/884849431820087297

This could seem a little odd seeing how Capaldi recently finished his last day on the Who set. That surely means his regeneration scene has been filmed and Talalay knows who’s taking control of the Tardis, right?

Well, not necessarily. The simplest solution to the puzzle may be the obvious one: the regeneration is being filmed in two parts, with Capaldi not needed for the second. That’s just what happened in previous regenerations, with the incoming Doctor’s role in the scene filmed once the outgoing actor has finished their work.

The two actors aren’t needed on the same set at the same time. In fact, when Colin Baker regenerated into Sylvester McCoy, Baker didn’t turn up to set at all: McCoy played both Doctors, donning a blonde wig to play Baker’s part.

https://www.youtube.com/watch?v=U7HMqJcBFZg

While we can be fairly sure Peter Capaldi’s replacement won’t rock up wearing an out-of-control grey headpiece, we’re less sure about what will actually happen in the Christmas special.

https://twitter.com/rtalalay/status/884696738786881540

The plot of the icy story has been kept firmly kept under Tardis lock and key, but David Bradley’s appearance at The First Doctor points to one convincing fan theory.

However, when it comes to Capaldi’s replacement, we’re going to have to keep asking the universe's oldest question: Doctor Who?