Earlier this year I went to LTUE (Life, the Universe, and Everything—a symposium for science fiction and fantasy writers). While there I listened to a presentation on creating effective villains. One thing in particular that I learned was that bad guys can’t be bad just for the sake of being bad. In other words, every story needs a villain (whether tangible or not) and that villain needs to be three dimensional. He needs a back story. He needs motivation. He cannot be doing bad things just because you need a bad character. He needs to be driven. And as is mentioned several times in the Writing Excuses podcast, the villain needs to be the hero of his own story.

And even if you never explain the back story, the complexity of your characters will come through in your writing. Because the more you know about them, the more it shows.