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.
To answer general q's: No trip to SDCC for me this year. No knowledge of 13. No idea about S11. Reading some books. Any recommendations?— Rachel Talalay (@rtalalay) July 11, 2017
That 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.
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.
However, when it comes to Capaldi’s replacement, we’re going to have to keep asking the universe's oldest question: Doctor Who?
Via Radio Times by Thomas Ling