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. … 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 — make her fly. Donna is magnificently self-centred — 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.
– Russell T. Davies, The Writer’s Tale