Speaking of our characters, finding out who they are, where they are, what they're in the middle of doing, is what brings about the beginning of your story.
Too often, characters in fiction fail to become distinguishable from others in their fictional world, except perhaps by name, gender, and possibly some physical description quickly forgotten by the reader and occasionally, by the writer as well. This blurring of characters is an all-too-common problem among neophytes, and often not-so-neophyte writers, usually because they are so intent on moving the plot along. These writers may have given labels and costumes to their people, but they have forgotten to provide them with flesh and voice, blood and breath. They have failed to imagine them properly, to create them fully rounded in all their dimensions.
Interchangeable characters don't belong in a novel, any more than interchangeable friends belong in your life. It's their differences that make one's friends interesting and valuable.
Characters are invented out of the people in our lives - friends, family, aquaintances, people we know only through gossip, people we've read about in the newspapers - and then transforms for purposes of our story. But, when we as writers create our characters, we have an advantage that we don't have in life; we can know *everything* about them. Unlike even those we are closest to, the people we create can't hold anything back from us; they have absolutely no privacy. It is our business to decide just how much of them we need, how much to reveal, how much to keep hidden, how much to allow the reader to imagine. In a novel, even in a story, we make a world, we make the people in the world, we make what happens to them; we dictate, out of the kind of people we have made them, their actions and reactions, and we go even further - we give, or at least we try to give, some meaning to it all (and this may be why we do it in the first place.)
Our characters, if we have pumped enough blood into them, develop minds and wills of their own. When this happens, when we surprise ourselves, we experience what I believe is the real joy of writing: The book, or story, has come alive, and we can be fairly confident that it's going to succeed. Our fictional people sometimes won't go where we send them or say the things we meant them to say. It's important, then, to listen to them carefully, to follow where they lead, to make the comprimises they demand of us, to trust them. If we have created a powerful person then we must allow him to exercise his power.
Of course, there is always the risk that a character, given her head, may bludgeon the book to death. If this happens, then we must decide which is at fault, our original premise for the book or something in the character that doesn't belong there and may have to be altered.
What a writer does when she puts herself into her characters is what actors do - not merely interpret the characters but become them. While we have only the baggage of ourselves to bring to these roles, that baggage holds much more than we can ever remember packing into it. Writing, letting the mind roam free, is like foraging through an almost bottomless old trunk in the attic; much of what we pull forth may be drab, moth-eaten, discardable, but we never know what bright, lovely unremembered treasure lies hidden waiting to be found. The great pleasure of writing lies in these discoveries.