When it was revealed earlier this year that David Bradley was bringing the very first incarnation of lead character the Doctor back to Doctor Who, fans were brilliantly surprised – and now it’s emerged exactly where the idea for the casting coup came from.

You see, while it now seems obvious that Bradley should be the one to revive William Hartnell’s Doctor (he played Hartnell on TV in for 2013’s An Adventure in Space and Time, after all), the thought had to come from somewhere – and it was series lead Peter Capaldi who apparently lit the spark in head writer Steven Moffat’s mind.

“I was at New York Comic Con and someone was asking me about the War Doctor in [50th anniversary special] The Day of the Doctor,” Moffat recalled on the set of the upcoming Christmas special, where Bradley’s First Doctor appears.

“I was talking about maybe the biggest contrast you could have between Doctors, because they are all the same person and obviously there is a limitation.

“I said, ‘Well the ideal one you want to do is William Hartnell meets the current Doctor, because that’s such a colossal difference; he's so different by that stage. Imagine how that lovely sweet old man, who isn't the least bit crotchety when you go back and look at him, would react to discovering he's this buccaneering space hero with the ego the size of a planet. How's he going to react? What's he going to say? But we can't do it because William Hartnell's dead.’

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“Peter said ‘We can get David Bradley,’ and I thought ‘Oh yeah, actually we could. We could absolutely do that.’ So we did.”

And Bradley was extremely happy to get the call, as he later explained to us.

“When we were doing [An Adventure in Space and Time] Mark [Gatiss] did say ‘Wouldn't it be great to find some old lost episodes and do them?’ ‘Yeah it would. It won't happen but it would be nice,’” he recalled.

“But I never thought for a moment that I'd be asked to actually do an episode, let alone the big Christmas one.

“When I was just playing Hartnell I couldn't say I played Doctor Who, I played William, I played the actor, playing Doctor Who.

“But now I can say I've been the Doctor,” he concluded. “And there's a figurine coming up to prove it!”

And all that momentous merch is thanks to the outgoing Twelfth Doctor himself, Peter Capaldi. Even when he’s leaving, he can’t help making the series great.

Doctor Who: Twice Upon a Time airs on BBC1 on Christmas Day (Monday 25th December) at 5:30pm