"It’s the fastest return ever on Doctor Who!"
Steven Moffat says he was aware that it was a bit "strange" to give Matt Smith's 11th Doctor a cameo in his successor Peter Capaldi's debut episode, but that "it just felt utterly right".
Smith's Doctor surprised viewers on Saturday night, appearing on one end of a telephone line in an emotional conversation across time with companion Clara (Jenna Coleman).
The scene served to reassure both Clara and viewers that Capaldi's newly regenerated Doctor was one and the same man as the younger model he had left behind, and Moffat said that was exactly the kind of thought the 11th Doctor would have had.
"It just felt utterly right for what we were planning for Peter’s Doctor – and right for Matt’s Doctor that he would think of that as he was just about ready to go out the door," said Moffat, speaking at the London premiere of the episode.
"And you think, well it’s never been done before – why not?"
The showrunner admitted he had briefly considered whether it was too soon a return for Smith's recently regenerated Doctor – but that he had quickly dismissed any qualms.
"I did wonder. I wondered when we sat in the cutting room, 'Does this seem strange?' And then you remember you’re making Doctor Who and you go, 'Yes this does seem strange – that’s absolutely fine!'"
Equally pleased at the decision was Smith's successor, Peter Capaldi.
"I love Matt, I think he’s fantastic," said the new Doctor. "He had a great kind of wisdom about him; he had this great combination of youthfulness and this very old soul, which is very unique, so I was delighted that he showed up."
The cameo appearance took place at the end of a rollercoaster debut for Capaldi, which saw him adjusting to his new face and companions and battling an old foe in the form of the clockwork droids. But Smith's scene had actually been filmed before all that, during his final full episode, the 2013 Christmas special.
"We shot that on the set of Time of the Doctor," said Moffat, "which meant I had to write that scene first and then fill in some gumph about clockwork robots."
The Doctor Who issue of the new Radio Times iPad and iPhone edition features an exclusive animated cover and photo galleries, a 68-page bonus magazine looking back at the 10th anniversary of Doctor Who, an interview with Peter Capaldi and Steven Moffat's series-eight episode guide