The past appearances of Peter Capaldi in the Whoniverse is a topic that has long been discussed and theorised since the actor's announcement as the Twelfth Doctor. Former showrunner Russell T Davies has even himself postulated a theory outlining the existence of these identical weathered faces. Now, Capaldi has offered the biggest hint to date of Caecilius's return from The Fires of Pompeii in an Apple Meet the Cast event:
“With regards to my Roman alter-ego, there will be a… he may appear again.”
“He will certainly be invoked.”Capaldi's language suggests at least a reference to Caecilius beyond the hints dropped in Deep Breath, but what with Davies's theory and current showrunner Steven Moffat's own admission of addressing the elephant in the room, Series 9 is shaping up as the likely destination for the end of this mystery.
[Source: Doctor Who TV]