We know that previous Showrunner Russell T Davies had a plan while he was working on the show and Moffat has stated that this will serve as the basis for the explanation.
Talking at London Film and Comic Con, Moffat said:
“Truthfully I don’t think it’s something you have to resolve because audiences do understand that the same actor can play different parts.
“When Peter Capaldi turned up in Torchwood Russell said he had a plan in his head on why he looked like the guy in The Fires of Pompeii. So I emailed him and said what was the explanation and does it fit with the new Doctor? And it sort of does.
“So in a very low-key way we’ll address it. It won’t be a major deal because in the end people know the real reason is he’s played by the same actor.
“What’s really worrying me is Karen Gillan in The Fires of Pompeii. That’s just inexplicable – I’m going to get to that eventually.”How do you think Capaldi's previous appearances will be explained? Share your theories in the comments below.