"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 thinking. Donna is blunt, precise, unfiltered, but with a big heart underneath 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-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."
Someone quoted this from his book which I can't want to get someday. But I agree that Martha was calm and quiet and that might've hurt her as a character because Freema in real life isn't like that. And I think Donna's bluntness is her greatest appeal she said what she felt and more importantly what she thought. Rose was certainly selfish and it showed even at the end of Journey's End.