What does it mean when a character in a novel or short story comes alive?
I imagine the following: some part of the human brain has the function of understanding other people we know, in order to assess their likely future behavior, figure out how to get on their good side, and so on.
When you're reading and writing, these interpretative and predictive functions are hijacked. If you're engrossed in a story, the neural networks activated in your brain are probably some of the same neural networks activated when you're trying to understand the people you interact with in your own life, and where their lives might be going. This last statement could be empirically tested, and perhaps has been -- right now I can only say that I believe it to be true, based on my own experience as a reader and a writer.
If someone we know does something that perfectly illustrates our idea of who they are, we delightedly share the story with our peers -- this is essentially the same joy that we take in the behavior of fictional characters. And when a character in a story I'm writing comes alive for me, that hopefully increases the likelihood that some percentage of my potential future readers might feel the same way. Here's Wilde on Balzac:
"A steady course of Balzac reduces our living friends to shadows, and our acquaintances to the shadows of shades. His characters have a kind of fervent fiery-coloured existence. They dominate us, and defy scepticism. One of the greatest tragedies of my life is the death of Lucien de Rubempre. It is a grief from which I have never been able completely to rid myself."
Graham Greene's The End of the Affair has an excellent passage about the irritation of a writer unable to coax his brain into fully engaging with a character:
"Always I find when I begin to write there is one character who obstinately will not come alive. There is nothing psychologically false about him, but he sticks, he has to be pushed around, words have to be found for him, all the technical skill I have acquired through the laborious years have to be employed in making him appear alive to my readers. Sometimes I get a sour satisfaction when a reviewer praises him as the best-drawn character in the story: if he has not been drawn he has certainly been dragged. He lies heavily on my mind whether I start to work like an ill-digested meal on the stomach, robbing me of the pleasure of creation in any scene where he is present. He never does the unexpected thing, he never surprises me, he never takes charge. Every other character helps, he only hinders."
"And yet one cannot do without him. I can imagine a God feeling in just that way about some of us. The saints, one would suppose, in a sense create themselves. They come alive. They are capable of the surprising act or word. They stand outside the plot, unconditioned by it. But we have to be pushed around. We have the obstinacy of nonexistence. We are intextricably bound to the plot, and wearily God forces us, here and there, according to his intention, characters without poetry, without free will, whose only importance is that somewhere, at some time, we help to furnish the scene in which a living character moves and speaks, providing perhaps the saints with the opportunities for their free will."
Canonization and personality cults are attempts to make many people fall in love with someone they don't really know: this is the same effect we writers strive to achieve with our protagonists. Perhaps this helps explain the following Harold Bloom quote:
"Novels require more readers than poems do, a statement so odd that it puzzles me, even as I agree with it."
Perhaps novels require more readers because we novelists want people to know the characters we've created, just as when we meet somebody interesting, we wish to introduce them to our circle of friends? Whereas poems concern more private, less character-driven experiences, hence stimulating a less socially-oriented part of the brain?