Creating a character is an evolutionary process. It doesn’t happen all at once, but incrementally. How could it be otherwise? Real life people don’t spring from the ground fully formed either; they live and grow. It’s that way with your characters, too. Let’s look at character growth for a moment.
Unless the story idea is character-centered, you’ll generally give birth to a character to suite a story premise. The character’s story role (protagonist, antagonist, sidekick, love interest), role (detective, housewife, high school outcast), sex and age are about all you need so start with. You could say that the character’s defined by its genetics.
As the premise develops, a protagonist’s or antagonist’s goal and motivation take center stage. You love a baby not because of its complexity but, in part, because of its potential. So with your character.
Genre and milieu determine, to a large degree, your character character’s background. Spaceship mechanic’ or ‘time machine inventor’ are science fiction, former outlaw turned sheriff is western, disillusioned stockbroker is mainstream.
But none of these are fleshed-out characters, or even roles. A character needs a back story of his own. And the back story needs settings and world-building to flesh it out. Character is not simply genetics, but also environment. As your world grows more detailed, so too will your characters.
Adolescence is that period of life when, to paraphrase Logan Smith’s, you ‘try on one face after another to find a face of your own.’ It’s a time fraught with problems, and as you fit your character to your story problems, and particularly inner problems- his character arc- individual personality begins to show. He begins to exhibit a voice of his own.
Your character is ready to step onto the story’ stage. At this point, goal, motivation, and conflict are key. Your character will be interacting with others and situations, will speak and act. Like a parent, some of that maturity you’ll recognize- it’s how you raised him. Like a parent, you should expect to also be surprised when his actions and dialog aren’t quite what you thought they’d be. Maturity, in characterization as in life, is independence.
This is your character at the end of the story- and at the end of his character arc. He may not be old in years, but one thing is certain: he should not be the same as he was at the tale’s beginning.