Peter Capaldi was cast as the Twelfth Doctor after he played another character in Doctor Who, but this casting trick was done first by Colin Baker's Sixth Doctor. On August 4th, 2013, the BBC revealed that the twelfth incarnation of The Doctor would be played by Peter Capaldi, known for his portrayal of corrupt politician Malcolm Tucker in The Thick Of It. However, Capaldi had also previously starred in Doctor Who as Pompeiian man Lobus Caecilius in season 4, episode 2, “The Fires of Pompeii."
Caecilius encountered David Tennant's Tenth Doctor just before the eruption of Mount Vesuvius in 79 AD. Initially, The Doctor left Caecilius and his family to die in the disaster, but his companion Donna Noble, played by Catherine Tate, convinced him to go back and save them. Thus, when The Doctor regenerated into Capaldi's incarnation just a few years later, showrunner Steven Moffat made it clear that the likeness was more than just a coincidence. In Doctor Who season 9, episode 5, "The Girl Who Died," Capaldi's Doctor remembered that he chose Caecilius' image when regenerating as a reminder to always try and save people.
However, this wasn't the first time The Doctor had regenerated into a familiar face. Almost three decades prior, Peter Davison's Fifth Doctor regenerated into the Sixth Doctor, played by Colin Baker. This was despite the fact that Baker had already played Commander Maxil in the Fifth Doctor adventure "Arc of Infinity," making it the original casting trick. Unlike Capaldi and Caecilius, the show never addressed the visual similarities between Colin Baker's Sixth Doctor and Maxil. Nevertheless, it had already been known for some time that regenerating Time Lords could somewhat control their new appearance. This was due to the Fourth Doctor's Gallifreyan companion Romana, who chose to regenerate into the image of Princess Astra, both played by Lalla Ward, simply because she liked her face. Therefore, there have been many unofficial theories as to why the Fifth Doctor chose to regenerate in Maxil's image, one of which relates very closely to Capaldi's casting trick.
For the most part, Davison's portrayal had highlighted The Doctor's softer, more peaceful traits. However, despite his pacifist stance, the Fifth Doctor still caused controversy across the universe and still lost people. Most notably, his companion Adric met his demise in "Earthshock." In sharp contrast to this, the Sixth Doctor was arrogant, more violent, and generally a much darker version of The Doctor than his predecessor. In this sense, he replicated the behavior of Commander Maxil, who wasn't afraid to use violence, even stunning the Fifth Doctor with a staser blast.
Thus, the Sixth Doctor's appearance could have possibly represented a reaction against Davison's version of the character. Indeed, the fact that he went as far as to attack his companion Peri following an unstable regeneration meant that Colin Baker's Doctor was seen as much less considerate and likable than Davison's portrayal. However, The Fifth Doctor's guilt for what happened to Adric might have been what made his new incarnation more violent, something which he ultimately did try to use in order to save the good guys. If that were the case, his likeness to Maxil would have served as a reminder of his previous incarnation's failures and how to rectify them.
Although Doctor Who's casting tricks took place almost thirty years apart and were handled very differently by showrunners, Capaldi and Baker's Doctors could both have been reminders to themselves about the importance of saving others. It could even be argued that the Sixth Doctor also represented an unconventional method of saving good people, as his face embodied a darker demeanor. Nevertheless, this behavior still turned out to be flawed, as he often came off as abusive and eventually led his companion Peri to her death in the first part of "The Trial of a Time Lord." In the end, whilst Capaldi's casting trick clearly suggested The Doctor's determination to save others, Maxil's image could have actually been foreshadowing the darker times ahead for the Sixth Doctor.