How does one achieve that level of believability when writing? I don't know the definitive answer to that, but I have an idea that might explain it. It isn't enough to just write about the characters and what happens to them. Anyone can write X went to the land of weirdos and met Y. They fell in love. The big bad prevented them from being together. They overcame. The end. You have to BE them. When I write, I get inside my characters' heads. I feel what they would feel. I write the dialogue I hear them say. I sense how they would react to their situations. When I close my eyes, I can see them, and I listen to them when they talk to me.
And, yes, they do talk to me. That doesn't mean I need a one way trip to the asylum either. What it means is they give me little tidbits of information about themselves that I have to build on. For example, Remy came to me in a dream and showed me a pivotal scene that happens toward the end of Craving, and then he wouldn't leave me alone about it. He kept shoving it to the forefront of my mind until I caved and decided to explore it. I currently have someone who keeps telling me his name, and I have no idea what he wants me to do with that information, but I'll figure it out. Let it be known here that Adam is an entity who will be explored at some point. Now let's see where he pops up in the future. He feels like a werewolf to me, but he hasn't told me that yet. Definitely supernatural though. I also have another vampire, more traditional than the vamps in Willow Creek (think Dracula-esque), who keeps making his presence known. I've tried to coax some things out of him, but he hasn't given up much yet. Who knows? Maybe he's Adam, and he's actually a vampire instead of a wolf.
Anyway, I got off track there for a minute. Back to my point, I'm often asked when I do interviews what piece of advice I would give to aspiring writers, and I usually say to just keep writing. But it isn't enough, is it? I think the better advice would be to just keep writing from inside your hero's/heroine's/villain's head instead of from the outside looking in, because THAT is how you make them interesting, and THAT is what will draw your readers into your world.