Under the tenure of Russell T. Davies, Capaldi starred as Caecilius in Doctor Who and John Frobisher in Torchwood, two highly significant roles to the episodes in which he appeared, and Moffat explained that he doesn't intend to ignore those appearances.
“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.”Moffat also explained his take on the changing faces of the Doctor each time he regenerates and how that might affect Capaldi's regeneration from outgoing Doctor Matt Smith.
“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?"[Source: Doctor Who TV]