Peter Capaldi

Peter Capaldi played the Twelfth Doctor from a brief cameo in The Day of the Doctor, taking over the role permanently in The Time of the Doctor. Like Colin Baker, he also appeared in a different role in Doctor Who prior to playing The Doctor, portraying both Caecilius in The Fires of Pompeii, and John Frobisher in all five episodes of Torchwood: Children of Earth.

Relationship with Doctor Who
During the Jon Pertwee years of Doctor Who, Capaldi sent scripts to the production office. Barry Letts began a correspondence with Capaldi, which eventually resulted in an invitation for the youngster to visit BBC Television Centre. Capaldi met Letts, Terrance Dicks and Pertwee. He later cited the incident as formative to his career as a professional actor, writer and director. (BBC DVD: The Monster of Peladon)

Capaldi was announced as the Twelfth Doctor in a live broadcast simulcast worldwide on 4 August 2013. His debut appearance during the climax of TV: The Day of the Doctor was kept a surprise from viewers as the BBC had indicated he would debut at Christmas. His initial appearance, lasting only a couple of seconds, reveals little other than his hand pulling a lever in his TARDIS and an extreme close-up on his eyes.

Other Notable Work
He is also the only person to play the Doctor (on a regular basis) who has won an Academy Award (John Hurt also had an Oscar, but was never the incumbent Doctor or the show lead). Despite best known for his acting, this was in the category for  starring Richard E. Grant and Crispin Letts (son of Barry Letts), which he wrote and directed. Indeed, Capaldi is the only actor to play the Doctor to have significant above the line credits on both sides of the camera prior to taking on the job.

Capaldi is perhaps best known to audiences as the foul mouthed Director of Communications, Malcolm Tucker, throughout the entire run of the BBC sit-com The Thick Of It, as well the spin-off film, In The Loop.