My literary agent recommended I look into astrology. That requires extra work, but in the end I found it to be worth it. I had to ask myself some questions I didn't consider. What month was the character born? What day? Which sign do they fall under? Astrological signs have been used for centuries to determine personality traits.
Next time you plot out characters, dig a little deeper. Figure out their date of birth, find out their sign and see what traits make these characters who they are. You never know what you might find.