Peter Capaldi's dual role as the Twelfth Doctor and Caecilius became a fully-fledged plot point, culminating in the Doctor realizing he had subconsciously taken Caecilius's form to remind himself ...

while Peter Capaldi showed up in ‘The Fires of Pompeii’ as Lucius Caecilius before playing the Twelfth Doctor. Outside of that, nothing has officially been confirmed when it comes to the ...