In terms of the companions all being ‘the same’ — that’s not as phony or artistically crap a thing to say as it sounds. What is the base group of people who would run away with the Doctor? They’re all going to be a bit mad. A bit dislocated. Not happy with where they are. Are they yearning for outer space? They’re going to be people who feel like they can take on the Doctor, who’s quite an intimidating sort of person. So, they’re going to be feisty — they’re going to be all those things. He sort of defines the people who are going to travel with him. The distinction comes very much from the various actors and actresses. So, you know, they’re the ones who create the differences between them.

Steven Moffat on companions