Steven Moffat is just weeks away from ending his tenure as showrunner on Doctor Who, and as the new Christmas special approaches he's making the rounds to promote it. Before he ran the series, though, Moffat was a writer who delivered some of the revival series' most beloved episodes, from "The Girl in the Fireplace" to "Silence in the Library." He also wrote 2007's "Blink," which many fans would call one of the greatest Doctor Who episodes of all time.
Part of the premise of the episode is that The Doctor and Martha have been displaced in time by The Weeping Angels (the infamous monsters who made their debut in this story), and therefore are having some trouble getting back to the TARDIS. As such, The Doctor communicates across decades with a young woman named Sally Sparrow, played by Carey Mulligan. Because The Doctor is sort of lost, Mulligan ends up carrying much of the episode in a great and endearing performance.
A great many accomplished British actors have wandered through Doctor Who over the years, often in pocket episodes exactly like "Blink." They make an episode, move on, and then get to say they were on a legendary TV series. Mulligan was one such actor, but according to Moffat, there was an opportunity for her to return as a full-blown Companion to The Doctor.
"Oh my God, Carey Mulligan! It’s funny, but 'Blink,' I say immodestly, is a very famous episode of television, and yet Carey Mulligan, who was the star of it, I’m almost certain wouldn’t even remember being in Doctor Who," Moffat told Radio Times. "I don’t think she was much of a fan, or anything. They liked her so much, they said, 'Do you want to be the next companion?' but she said no. God, she was amazing."
Mulligan's gone on to have a very diverse and celebrated career, with roles in everything from indie hits like Shame to more glamorous fare like The Great Gatsby (you can currently see her in Mudbound). As for the role of the next Companion, it went to the great Catherine Tate as the relentlessly feisty Donna Noble, who, funnily enough, had also appeared on a one-off episode (the 2006 Christmas special "The Runaway Bride") before landing a regular role on the show.
There are certainly many fans who would have loved seeing more Sally Sparrow in The Doctor's life, but when asked if he ever pressed Mulligan to return to the show, Moffat said that he both knew she'd decline and understood that there's something special about letting her exist in only a single story.
"But then does that character become more special because you never see her again? She just passes through the Doctor’s life," he said. "It’s surprising the people who do love being part of it. John Hurt loved being the Doctor and was quite insistent, asking, 'Am I a real Doctor? Do I really count?' And we said, 'Yes, you count. You’re on the poster. It is definitely you.'"
Doctor Who returns with its annual Christmas special, "Twice Upon a Time," on Christmas Day.