The revival of BBC's Doctor Who brought some significant changes to the titular character, the Doctor. Since returning in 2005, the character became younger and quirkier with each regeneration until Peter Capaldi was cast in the role in 2014, returning a seriousness to the Doctor. In keeping with this new direction, Capaldi also brought the same heavy thoughtfulness to his departure from the role.
Peter Capaldi's Twelfth Doctor was a real change of pace. Not only did he bring age and experience to a role that had gotten progressively younger over the last three actors, Capaldi also kept his Scottish accent for the role. Coupled with a stark jacket and collared shirt, the end result was a Doctor who was much more serious and aloof than previous versions. Capaldi wanted his doctor to feel more alien, putting more distance between himself and his human companions.
Capaldi took over the role of the Doctor from Matt Smith in 2014. He makes an uncredited special appearance in the 50th anniversary special "The Day of the Doctor," but is fully revealed in the next episode "The Time of the Doctor" as Matt Smith's Doctor is granted another regeneration. Strangely, Capaldi had also previously appeared in the series as other characters, playing Lobus Caecilius in Tennant-era episode "The Fires of Pompeii" and a civil servant in an episode of the Doctor Who spin-off Torchwood.
Like the two Doctors before him, Capaldi stayed on for three seasons before announcing in 2017 that the tenth season would be his last. Similar to previous Doctor Smith, Capaldi claimed that it was the right time for him to leave the show rather than citing any specific incidents or complaints leading to his departure. But Capaldi also described how self doubt impacted his decision, and that along with the draw of new roles was the worry about complacency and becoming too comfortable in one part. "That's not what being creative is," Capaldi declared to crowds at New York Comic-Con.
Capaldi's timing was fortuitous, as showrunner Steven Moffat had also announced his departure from the series. Moffat had been in charge since Smith had taken over the role of the Doctor, bringing a new feel to Doctor Who with a clean slate of brand new showrunner and brand new Doctor. With Moffat and Capaldi departing at the same time, they offered a similar clean slate to those who would be taking over the show from them -- Thirteenth Doctor Jodie Whittaker and new showrunner Chris Chibnall.
While Capaldi left his role of the Doctor for many of the same reasons as the actors before him -- his time had run its course and he was ready to play new roles -- he also brought an introspection to this decision. For Capaldi, leaving Doctor Who was about ensuring he was always bringing creativity to his roles, demonstrating that the actor shared the seriousness that was characteristic of his version of the Doctor.