From another book I came across, with a more philosophical approach, since we're discussing rules and Loki asked about characters.

'The novel is the great virtuoso of exceptionalism: it always wriggles out of the rules thrown around it. And the novelistic character is the very Houdini of that exceptionalism. There is no such thing as "a novelistic character." There are just thousands of different kinds of people, some round, some flat, some deep, some caricatures, some realistically evoked, some brushed in with the lightest of strokes. Some of them are solid enough that we can speculate about their motives ... But there are scores of fictional characters who are not fully or conventionally evoked who are also alive and vivid. The solid, nineteenth-century fictional character ... who confronts us with deep mysteries is not the "best" or ideal or only way to create character (though it does not deserve the enormous condescension of postmodernism). My own taste tends toward the sketchier fictional personage, whose lacunae and omissions tease us, provoke us to wade in their deep shallows ... We have only their unreliable narration of events.'