It's almost time to bid farewell to Peter Capaldi's Doctor – and he'll be going out with a bang, not a whimper.

Doctor Who's head writer and showrunner Steven Moffat has hinted that Capaldi's regeneration – coming in this year's Christmas special 'Twice Upon a Time' – will be the most explosive we've ever seen.

BBC

On set for the special, Digital Spy asked Moffat why this Doctor is fighting so hard to avoid his latest face swap. "I think he's just tired of it," he explained. "We go into that in this episode.

"Imagine you had to regenerate? It's not dying. It wouldn't be like dying at all. Dying's awful – you just disappear. But [with regeneration], you really would be somebody else. Imagine that?

"Imagine you had to go through a process, and you had no idea how you're going to come out of it. You look in a mirror, and you feel different, and you sound different. It would be absolutely terrifying."

But according to Moffat, there's another reason why the Doctor is scared to change – each time he goes through the process, his regenerations grow more violent and less predictable.

BBC

"I was worrying about this with Chris [Chibnall, taking over as showrunner in 2018]. Every time we regenerate him, he's blowing up more and more shit, isn't he?

"He used to lie delicately on the floor and wibble a bit, and he'd be somebody else. Then [in David Tennant's swansong, 'The End of Time'], he blows up the whole entire TARDIS – for no reason that either Russell [T Davies] or I could think of, but someone on the internet suggested that it was because of all the radiation he'd absorbed.

"Then [in Matt Smith's final outing, 'The Time of the Doctor'], he blows up an entire Dalek fleet. I mean, he must be worried he's going to wipe out half the universe this time. So we have included the idea that it's getting more volcanic each time."

Related: Could Doctor Who recast more old Doctors? Steven Moffat thinks it's possible

Of course, we know that he eventually goes through with it – with Capaldi giving way to Jodie Whittaker. Helping our current Doctor through his regeneration crisis is his very first incarnation (David Bradley), who's pondering a similar problem.

BBC

For the first Doctor, 'Twice Upon a Time' takes place towards the end of the character's own final adventure, 1966's 'The Tenth Planet' – with an unused line from that story's original script helping to inspire Moffat.

"There was one particular line they didn't use, which is the Doctor saying, 'I can't go through with it. I'm not doing it'. It's not in the finished cut, but it was in the script. He's momentarily deciding not to [regenerate], because he's afraid. He's going through the same crisis."

How will the two Doctors overcome their regeneration fears? And what will cause the Time Lord's latest transformation? Just 19 days till we find out...

Doctor Who: 'Twice Upon a Time' will air this Christmas on BBC One.

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