The thing that hit me last night is how important it is to write the first line of dialogue for your character. In that line is everything. Yes, you will develop this character further but as soon as you write down his words -- he exists. In fact, it's shocking how fulsomely he exists.
This happens because in that one line of dialogue is everything you need to know. (Okay, not really but stay with me.) You commit to the character when you write a line of dialogue, and instantly you know this person. It's as if the line is DNA that contains the blueprint for everything that must follow.
I think this happens because of the nature of writing and the nature of humans. When you write dialogue, you're not listening, as you do in life when someone is talking. You're speaking through the character. And in doing so, you are him. I think this happens in large part because the writer can literally hear the character's voice as he speaks. And in that intonation, that delivery, that voice, is the character, whole and true. At first, only the writer can hear this voice but in the course of the book, he makes the reader hear it too -- and hear it truly.
The knowledge a writer feels comes also from human knowing, from the built-in talent we homo sapiens have for understanding the actions of others. It's that mirror neuron thing, when you get right down to it. By speaking in the character's voice, the writer becomes the character. This is nothing new for us. We humans can slip into the minds of others at will. We do this to figure out the motivations of others so we know whether to trust them or not. To do this, we in a sense become the other person, and in doing so we can feel if their words and actions line up in a logical fashion. We ask if they are believable, and we're uncanny in our ability to ferret out the truth. It's how we come to understand others and it's one of our basic survival skills.
So when we slip into a character, we learn a lot. It's not so much a writerly skill that comes into play -- but a human one. This is how characters are built: we get inside them and become them. And the moment you write that first line of dialogue, you've crossed the Rubicon. You're off and running. This is how characters come to be -- at least, in my worlds.