And now, Doctor Who Magazine has revealed how the regeneration scene was filmed.
Whittaker's regeneration scene was shot on 13th October 2021, while Tennant's took place seven months later "on a similar, slightly smaller green screen just up the road at Roath Lock Studios", the magazine revealed.
The Power of the Doctor’s director Jamie Magnus Stone was filming in Australia, so Rachel Talalay picked up the torch for Tennant's scene.
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“I can’t imagine anyone better,” Stone told the publication. “I’m an enormous fan of Rachel’s episodes. I think she’s the best director Doctor Who has ever had.”
On filming the moment, Talalay said: “I am so lucky I get to do this. That I get to re-regenerate David Tennant is phenomenal. I was already incredibly interested in returning.
"Just the fact that Russell [T Davies] was coming back was enough. When I was told it was David too, that took me to a whole other level.”
Speaking last month about his return to the sci-fi franchise, Tennant explained that "it didn't really feel like a risk".
"I knew that Russell T Davies was in charge and I love working with him and I love receiving a script with his name on the front, so that just felt like it was a bit of a no-brainer, really," the actor explained
Doctor Who magazine issue 584 is out now.