I think we all have a little built-in simulator in which we run miniature copies of all the people in our lives. These are the brain equivalents to computer games like The Sims. When you get to know someone, you put a copy of them in the simulator. This allows you to model their behavior, and thus to attempt to predict it. The simulator lets us guess which of our fellow humans is likely to be trustworthy, which ones might mate with us, which ones might beat us to a pulp if they get the chance.

That's Cory Doctorow on where fictional characters come from.

This, I think, is what happens when you write. You and your simulator collaborate to create your imaginary people. You start by telling your simulator that there’s a guy named Bob who’s on the run from the law, and the simulator dutifully creates a stick figure with a sign called ‘‘Bob’’ over his head and worried look on his face. You fill in the details as you write, dropping hints to your simulator about Bob, and so Bob gets more and more fleshed out. But the simulator isn’t just adding in the details you tell it about: it’s guessing about the details you haven’t yet supplied, so that when you go back to your imagination and ask it about Bob’s particulars, some of those answers come from the simulator – it’s a kind of prejudice that affects imaginary people, a magic trick where your conscious and subconscious minds vie to fool each other with compounded lies about fake people, each building on the last in a feedback loop that runs faster and faster as you go.

That’s why your characters eventually ‘‘come to life.’’ Eventually, your characters’ details contain so much data gleaned from things the simulator ‘‘knows’’ – because it has supplied them, after guessing about them – that they come to seem real to you, and to it (which is the same thing).