About two months ago, I wrote a post on creating well-rounded villains.
Well, although it's entitled 'Some Tips for Creating Well-Rounded Villains', it doesn't come off as tips, but as 'you must do all these things or you will have a lame villain'.
So, let's backtrack a little.
I would say that the only two things you really, actually need for a successful villain are: a motive, and average (at least) intelligence. In my mind, these are non-negotiable in a serious story. We're not talking about comedies where a ridiculously stupid villain still somehow takes over the world.
Why these two things are important is simple: believability. To me, having a villain who doesn't have a motive (power, revenge, etc.) and/or is brainless, but most especially the latter, annoys me. Sure, a story can still be entertaining if the villain is lame (think, old cowboy movies), but it might have me banging my head every time he shows up.
So, while the other 'tips' I wrote about are good to think about, they're just tips; take 'em or leave 'em. After all, the ultimate villain is the Devil, and while he definitely has a motive and is super intelligent, he doesn't conform to anything else on the list.
