Doctor Who's Steven Moffat has teased that Peter Capaldi's exit will be "different."
The 55-year-old show runner has been working with his replacement Chris Chibnall, who will take over the reins from next year, on the festive special and has hinted that the 59-year-old actor's regeneration into a new incarnation of the Time Lord - who will be played by someone new - will be something that viewers have never seen before.
Asked by Samira Ahmed at a special Hay Festival edition of BBC Radio 4's "Front Row" for "clues" about the upcoming festive special, Moffat said: "You haven't got to the end of this series yet, which ends on - I hope - quite an unexpected note.
"You all know that the mighty Peter Capaldi will be bowing out. But we're going to do it slightly differently this time. And I've been working with Chris [Chibnall] about how we do the changeover in a new way.
"So I'm not going to tell you what that is - I'm excited by it, I think it's going to work well - but it's not... well every regeneration is different, but we are playing it slightly differently this time. I think we've got a good idea - but I'm not telling you what it is."
It's not yet known who will take over from Capaldi but Chibnall - who is known for his work on "Broadchurch" - is planning to write the script before he picks his lead.
He said: "We'll cast the role in the traditional way: write the script, then go and find the best person for that part in that script. You couldn't go out and cast an abstract idea."
Some viewers have called for a woman to be cast as the Doctor and, although he's not giving it much thought at the moment, Chibnall has "a very clear sense" of what he wants.
He said: "The creative possibilities are endless, but I have a very clear sense of what we're going to do, without even knowing who's going to play the part."
Meanwhile, Capaldi has portrayed the time-travelling scientist since 2013 after he replaced Matt Smith as the titular character but he has admitted he always knew his stint would be relatively short because he doesn't like to do one thing for too long.