Here's who Steven Moffat originally planned to replace Jenna Coleman as the new Doctor Who companion

Moffat admits that he sensed at various points that Coleman may not yet have been ready to go, but he of course had to take her at her word, and even in the early stages began to turn his attention to the prospect of a new companion.

The eventual casting of Pearl Mackie as Bill Potts proved a huge hit, but Moffat has now revealed that way before that, when he expected Coleman to leave at the end of Last Christmas, he had a different replacement in mind: a return for a new character from the festive episode – Faye Marsay's Shona.

Marsay's brilliantly down to earth performance as the ordinary-yet-extraordinary shop worker/fantasy polar scientist prompted calls from fans for her to become a full-time companion. And it seems Moffat had the same idea.

"I was eyeing up the possibility of writing the part of Shona as the replacement," Moffat told Doctor Who: the Fan Show in the recently released third part of his exit interview – adding that Shona and Bill shared some key characteristics.

"There’s a commonality between the two," said Moffat. "I knew where I wanted to go next was a kind of earthy irreverence, I guess, if I put it in the most basic terms. I hadn’t got a fully formed character but I was messing around with that."

But it was at that point that things got emotional for Coleman.

"I had already persuaded her by the [time of the Doctor Who] World Tour to come back and do Christmas and ‘I’ve got a big ending for you where you turn out to be an old lady and you die at the end as an old lady," said Moffat.

"That was the original ending to Last Christmas and the ending we had at the read-through[...], and after the read through we went into a side room with Jenna where she was just… she wasn’t ready to go. She thought she was ready to go and I think in my heart I knew she wasn’t ready to go and she said ‘I’m not sure I’m actually ready to go’. And I said ‘well if you’re not ready to go from a job like this and a show like this, you shouldn’t go. It’s too damaging, it’s too emotional, it’s too big to leave before you’re ready, you mustn’t do that.’”

The rest, of course, is history. And given that it resulted in a great final series for Clara, and the debut of the immediately popular Bill, it can only be seen as a good thing.

But that doesn't stop us wondering what might have been had Shona got the chance of a full stint in the Tardis.

And if so, whether the Doctor would have allowed any more of this...