I've read all the advice about how to create good characters, and I've put it to use. I write backgrounds for them, read their dialogue aloud to be sure it sounds authentic, pay attention to the changes that they go through as the story progresses. But there's one more thing that has to happen, and I haven't got a lot of control over it: they have to grow.
Like children under the old tabula rasa theory, characters are blank when we create them. "I need a guy who can tell the other guy about the murder. He's a local and he's trustworthy."
Well, that's a beginning. But who is he? What's his function? Why does he tell what he knows, and why to this person?
Even when those questions are answered, sometimes even when the first draft is written and the information has been imparted from Character A to Character B, there is another step. That local guy has to become a person, and I have no idea how that happens. Time is the best answer I've found, so I often leave projects alone for days or weeks, until that guy's "personhood" solidifies in my mind and I can describe him well enough to suit my story's needs.
Some characters are people right away; others take a looooong time to capture, and maybe I never do capture others. Shakespeare certainly had throwaway characters, people who had no depth because he didn't care and didn't need us to. But those worth some consideration have to sit inside my head for a while, growing from flat to fleshed, from simply named to noteworthy.
Of course, that's followed by finding the right words to make the reader see the character grow into a person as well. Maybe we'll tackle that tomorrow.