Doctor Who head writer Steven Moffat noted that even though Peter Capaldi has appeared in previous episodes of the show, those appearances will not be ignored once he takes over as the new Doctor, hinting they might serve a plot points for future episodes.
During an interview with Nerd³, Moffat said: “We are aware that Peter Capaldi’s played a big old part in Doctor Who and Torchwood before and we are not going to ignore the fact.
“I remember Russell [T Davies] told me that he had a big old plan as to why there were two Peter Capaldi’s in the Who universe: one in Pompeii and one in Torchwood. When I cast Peter and Russell got in touch to say how pleased he was, I said, ‘Okay, what was your theory and does it still work?” and he said, ‘Yes it does. Here it is…’
“We’ll play that one out over time. It’s actually quite neat.”
He also added: “The face is not set from birth. It’s not like he was always going to be one day Peter Capaldi. We know that’s the case because in The War Games he has a choice of faces. So we know it’s not set, so where does he get those faces from? They can’t just be randomly generated because they’ve got lines. They’ve aged. When he turns into Peter he’ll actually have lines on his face. So where did that face come from?”