Doctor Who: 10 Things You Didn't Know About The Fourteenth Doctor
6. His First Line Was Improvised
Many fans correctly guessed that The Power of the Doctor would end with Jodie Whittaker regenerating into David Tennant rather than Ncuti Gatwa.
Some even predicted that it would end with Tennant exclaiming “What? What?! WHAT?!”, just as he’d done at the end of Doomsday and Last of the Time Lords. "It wasn't exactly the longest day at the keyboard," RTD later joked.
One aspect of the regeneration that was a surprise was Fourteen’s first line, “I know these teeth!” – a reference to the end of The Parting of the Ways when the newly-regenerated Tenth Doctor remarks that his new teeth feel “weird”.
The line even came as a surprise to RTD, because he didn’t actually write it. Instead, it was added by Tennant himself, as a tribute to his first ever Doctor Who scene (presumably he drew the line at namedropping Barcelona too).
As such, for the first time in the show’s history, an incoming Doctor helped pen their first scene – which, given the fact that Tennant had played the Doctor before, and that he’s such a big Doctor Who fan, feels very apt.