There's an old cliché that says you should "write what you know." I've always thought that was complete bullshit. Not many people have found themselves wandering through the terrifying aftermath of a society wrecked by a nationwide EMP, or invented a machine that could drill to the center of the Earth. Had William Fortschen of Jules Verne decided to stick to "what they know," we'd have never had the pleasure of reading their work.
What I prefer is to imagine yourself in a fantastical scenario. Why? Because it's much easier to envision the way a story would unfold if you are a part of it. Although they morph throughout the story, all of my main characters begin as me. I try to think, "What would I do if I hunted vampires for the Catholic Church?" or "Would I seek revenge if a bunch of eco-terrorists murdered my family?" Employing that device allows me to advance a story and move from plot point to plot point.
Some of you might be able to imagine a character out of scratch, as well as the tale that follows. Me, I simply can't - I'm not that broad. I have to be inside the head of the character to understand why things happen the way they do. As I've said before, I'm not the most socially graceful person in the world. Although not as awkward as Sheldon from The Big Band Theory, I'm not a social savant who blends in effortlessly with people and automatically empathizes. It takes me some work to figure folks out, and I can best envision my stories if I place myself in the middle of them.
I think there are more writers that use this device than would care to admit. Stephen King admitted that Jack Torrence was taken from the antagonism he felt for his own children, and that helped make the man more real to those of us who enjoyed his book. By putting yourself in these circumstances, you give your characters depth that you'd only be imagining if you didn't understand them. I know that when I read a book, I can tell which characters were thrown together by the author as products of pure imagination by the level of shallowness they present. No, I don't mean Paris Hilton type of shallowness, but the kind of shallowness that results from characters not well fleshed out. Experience provides depth, and we are our own best gage of that experience.
So figure out what you'd do if you had run of the world. What kind of story would you like to be a part of, and then envision yourself in it. Would you like to be on the bridge of a large starship as it hurtles through the galactic core, or would you rather be on the trail of a serial killer as a police detective? What would you do if confronted by a snarling werewolf, and what would your response be to the questions posed by a mentoring wizard? These questions form the basis for a great story, and our responses provide them believability.