To the general television audience, most people wouldn’t know her from any other young, beautiful Scottish actress. But “Doctor Who” fans should remember her from an episode last season. That’s because for the third time, producers have looked to its past guest cast roles to pick a new companion for The Doctor, this time around being played by Matt Smith. And the winner this time? Try 21-year-old Karen Gillan. “When she auditioned alongside Matt we knew we had something special,” said Piers Wenger, head of drama at BBC Wales, which produces the iconic series. Gillan doesn’t have a lot of acting credits to her name outside of a handful of British shows. But she did appear as the Soothsayer in the Season 4 episode “The Fires of Pompeii,” and apparently impressed the production team even …