Russell T Davies, when asked about how he writes the different companions on Doctor Who:
"That's tricky. I don't type 'DONNA' and then think, now, how would she say this...? The fact that I've typed 'DONNA' means that she already has something to say. You can worry too much about speech patterns, about imposing different styles on the words, one for Rose, one for Donna, one for Martha, one for Sarah Jane. They're all women, on the side of good, in a sci-fi world, so their speeches aren't going to be radically different. It's not so much what they say, as why they say it and when.
"But I suppose there's a basic characteristic that I bear in mind. An essence. Rose is open, honest, heartfelt, to the point of being selfish, wonderfully selfish. Martha is clever, calm, but rarely says what she's really thinking. Donna is blunt, precise, unfiltered, but with a big heart beneath all the banter. But we come back to what I was saying ages ago about 'turning' characters. If Rose can be selfish, then her finest moments will come when she's selfless. If Martha keeps quiet, then her moments of revelation - like her goodbye to the Doctor in Last of the Time Lords, or stuck with Milo and Cheen in Gridlock - make her fly. Donna is magnificently self-centered - not selfish, but she pivots everything around herself, as we all do - so when she opens up and hears the Ood song, or begs for Caecilius' family to be saved, then she's wonderful."
- from "The Writer's Tale", http://www.thewriterstale.com/