William Hartnell was the first actor to lead Doctor Who when the show began in 1963, but it took Matt Smith's Eleventh Doctor to bring back one of the character's most underutilized character traits. As the first of all the actors to play the Doctor, Hartnell helped kickstart the Time Lord's now-vast lore, including the ability to regenerate, which was introduced at the end of One's reign. By the time Smith was cast, the franchise canon was bursting at the seams, but that didn't stop showrunner Steven Moffat from reminding the audience about a key fact of the Doctor's character.
Daniel Bibby
Daniel Bibby