Secrets are the dark side of our portraits. The Masters, in oil, and later photographers on film, used less light on one side of the face or subject to bring out depth or dimension. It’s how they created the realism of three dimensions when, as you know, the paintings were two-dimensional rectangles, same for film. They call it modeling. It makes a picture more interesting, less flat. In writing, characters need shadow too. Only in this case the shadow comes from within. The source of this darkness is usually the secrets a human has but shares with no one but themselves. The kinds of things that only self-love can abide. The literary opportunity here is that these very same secrets could also generate self-loathing.
In photography, contrast ratio is how much light is employed against how much dark. In literary characters, how much light they emit is also a ratio between their secrets, baggage and internal weight – against their lighter natures. This is a very essential tool in deep character analysis. That analysis, by the way, is always best done after the character has taken form. These character elements should be discovered as you are writing, not engineered into the DNA before you write. That way these foibles’ become more organic to the flow of the story and don’t stick out like… “And now a word from our sponsor, the character building department.”
So shading a character in prose is akin to utilizing “Rembrandt Lighting” in film or photography. Too heavy a hand, too much obvious contrast and we start to look stagey, over done. But the right balance of contrast and dimension brought on by the shadow of secrets will fit seamlessly into the canvas of the story.
Way back in 1930 the biggest show on the air, the radio air that is, was a show that started with the chilling refrain, “Who knows what evil lurks in the heart of men? The shadow knows…”