Short & Junk: Villains in Fiction

lil_coyote asks: How do you go about making a villain that is believably evil enough to give the hero a run for their money while still being someone the reader has interest in and wants to read about? You’ve set some epic villains loose in your plots, but they don’t overshadow the heroes. I suppose I’m looking to make sure my bad guy won’t be flat without making him so big that he takes over the story.