E. M. Forster, in his 1927 book “Aspects of the Novel,” presented the concept of “round” versus “flat” characters. He generally preferred the former to the latter except when the purpose was to arouse feelings of “humour” or “appropriateness,” maintaining that flat characters, comprised of a single idea or “factor,” could often be summed up in one sentence, such as Mrs. Micawber in “David Copperfield”: “I never will desert Mr. Micawber.” When a character possesses more than one factor, “we get the beginning of the curve towards the round.” …
In truth, Forster provided a hint or two, in his talk of “deeper” qualities, such as the deeper moral sense of Lady Bertram in “Mansfield Park,” and the capacity to be “surprising in a convincing way.” But there’s still a bit too much airiness in all that to provide a practical technique.
By giving a character something to hide — a secret — we create the illusion of depth: interior and exterior, seen and unseen.
If we believe someone is hiding something, we can’t help but pay more attention to him. Few drives are as strong as the one to find out — ask Pandora, or Psyche or Bluebeard’s wife.
Secrets need not necessarily be shameful, though many are, sometimes unreasonably. But they always speak to an aspect of what has happened to us that we can neither forget nor share — which, in fiction, creates tension between a character’s inner life and her dramatic interactions with others.