Choices under pressure reveal a character's essential nature
Pressure is necessary for change.
The most fundamental way to reveal a character’s nature is to put them under immense pressure. Force them into a dilemma and have them make a choice. Difficult choices reveal our priorities, our outlook, our view of ourselves and others.
“True character is revealed in the choices a human being makes under pressure—the greater the pressure, the deeper the revelation, the truer the choice to the character’s essential nature.”
Robert McKee
Cool action, fun lines, humor—all are good, but it’s pressure that gets to the heart of a character, which is where a reader want to go. You want to give me insight into who I’m reading about, but that isn’t possible with endless monologue or internal dialogue. It’s done with choices.
Necessity of risk
Even casual choices can reveal something about a character, but risk is a vital ingredient to understanding and revealing character. What is a character willing to lose? What tradeoff will they make? These are what make characters full, real, and contradictory.