I’m not sure I entirely trust the veracity of this Popbitch story (if there had been any plans to bring back Sally for series four, wouldn’t we have known about it from The Writer’s Tale?), but it’s still amusing :

Carey Mulligan was offered the role of Dr Who assistant,
which eventually went to Catherine Tate. She apparently
said no because Dr Who fans are too mentally obsessive.

You’ve got to admit, she’s got a handle on us there.