Orson Scott Card puts it this way:
One of the reasons fiction exists at all is to deal with that fear of inexplicable change, that uncertain dread that lurks in the background of all our human relationships. Because fiction lets us see people’s motives, the causes of their behavior, these stories about made-up people help us guess at the motives and causes of real people’s behavior.
Robert McKee takes this principle further. He suggests that if your character doesn’t change emotionally in every scene, you need to ask yourself if the scene belongs in your story. Here are some examples that he uses:
Obviously, we’re talking about degrees here, otherwise we’d have a farce on our hands. McKee uses the example in Casablanca to show how subtle shifts can create great tension. It’s the scene when Ilsa comes up to Rick’s rooms after having met him in the café earlier in the evening. She begins the scene wanting to explain to Rick why she abandoned him in Paris all those years ago. He begins the scene bitter and sarcastic. He has a low opinion of her. Everyone remembers how the two characters change in that scene, but it’s worth going back and viewing it, or you can read McKee’s analysis of the screenplay in his book, Story.
How do characters change? The writer subjects them to the three Ts: Test, Torment, Transform.
What is the character’s greatest fear? Make him face it. What does he dread? Avoid? Ignore? Who does he love? Hate? Make him face the things he avoids. Question everything he loves and believes in. Review your character’s strengths. What would happen if you took that strength from your character. What would he be left with?
Think of these tests like a refiner’s fire, and your character the silver that’s placed in the fire to burn away all the impurities. What you’ll end up with is a transformed character.