Before he was cast as the Twelfth Doctor in 2013, Peter Capaldi had already starred on Doctor Who in an episode back in 2008 opposite David Tennant. Though he’s not the only Time Lord to first act as a guest star on the show – Colin Baker played another role before he was cast as the Sixth Doctor in the classic series – this fact drummed up a lot of confusion from fans at the time. In response, showrunner Steven Moffat promised he’d explain it. And explain it he did.
To begin with, let’s familiarize ourselves with Capaldi’s first Who character, Caecillius, a kindly Roman man who appeared in “The Fires of Pompeii.” Though he was fated to die along with the rest of the city, the Doctor decided to save him and his family from the eruption of Vesuvius. Fast forward to 2014 when the Doctor discovers his new face for the first time. He knew he’d seen it before, but couldn’t remember where.
A year later though, in 2015’s “The Girl Who Died,” the Doctor finally remembered. At the time, he was grieving over the loss of Maisie Williams’ brave Viking girl Ashildr. Upon figuring out he was wearing Caecillius’ face, the Doctor realized that he’d subconsciously chosen it as a reminder to himself to always save people no matter what. He then set about finding a way to resurrect Ashildr.
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Interestingly, though, no mention was made of Capaldi’s third role in the Whoniverse. In 2009’s Torchwood: Children of Earth, the actor portrayed John Frobisher, a British politician whose life is ruined by an alien invasion by the child-feasting 456. In a tragic twist, Frobisher is forced to kill his family to protect them being handed over to the race.
Frobisher may be a distant descendant of Caecillius. Or, as ScreenRant suggests, perhaps the Doctor was just trying to tell himself that everyone needs saving, even those who commit terrible acts.