Carey Mulligan, who played Sally Sparrow in Blink (2007), was asked to be a companion to the Tenth Doctor – but turned it down!
Many may hold up Blink as a modern-day classic… but star, Carey Mulligan apparently doesn’t! Writer, Steven Moffat recently revealed:

“Blink, I say immodestly, is a very famous episode and yet Carey Mulligan, who was the star of it, I’m almost certain wouldn’t even remember being in Doctor Who. I don’t think she was much of a fan. They liked her so much, they said ‘do you want to be the next companion?’, but she said no. She was amazing.”

This would presumably have been after Freema Agyeman’s Martha Jones left the series; then-showrunner, Russell T. Davies had planned to introduce a journalist called Penny for Doctor Who Series 4, but Mulligan was seemingly so good as Sally Sparrow (and the episode proved an immediate success), she was asked to come back as a companion. In the end, Davies settled on Catherine Tate, returning as Donna Noble from The Runaway Bride (2006), and even though I like Blink, and Mulligan, I’m glad Tate got the chance to prove what an incredible actress she is.
As for Penny… She appeared in Partners in Crime (2008), was treated very poorly by all and sundry, and attempted to report the Doctor and Donna for lunacy. Fair enough.
Now, this is just a guess based on Moffat’s quote; he makes it sound like it was before he took over as showrunner, and so the only opportunity would’ve been for Series 4, or the 2009 Specials at a push. Then again, in his The Writer’s Tale book – highly recommended! – Davies doesn’t mention anyone except a companion called Penny, and Donna Noble, although he thought Tate would be far too busy to spend a year in Cardiff filming in the TARDIS!
So what do you think, dear readers? Would you have liked to see Carey in the companions’ line-up? Or is she best left as a well-developed one-off character?