Where do you get the ideas from, when creating personalities for your characters? How to Make Your Characters Believable?

For me, creating characters and their personalities comes organically. Stephen King wrote in his wonderful book On Writing, that stories are like fossils, waiting for the writer to unearth. I wrote my novel, The Osprey Man, this way. I had the general idea of what my story was, and began writing from the point of view of twelve-year-old Jacob, who has just learned that his best friend is dead when the novel opens. I unearthed what happened to him and his buddies as I wrote and revised the book.