Chuck Wendig has 25 things you should know about antagonists:

Antagonists who do evil just to do evil are basically big fucking cartoons. They’re Snidely Whiplash. They’re Cobra Commander. They’re Pageant Moms, Nancy Grace, Rush Limbaugh. In other words: boring, unbelievable, and totally untenable. Give them motivations beyond “being the biggest dick I can be.” Yes, you can in certain modes and stories get away with this (see: Batman’s Joker, or nearly any killer in slasher films), but it’s hard, and it puts an even greater weight on the shoulders of the protagonist.

One of the challenges of screenwriting is finding the sweet spot between two opposing needs:

  • You want your protagonist to be pushing the story, and not just responding to problems.
  • You want your antagonist to keep making life as difficult as possible for your hero, constantly raising the stakes.

Basically, your protagonist and antagonist each want to drive the plot. They’re fighting for control of the steering wheel.

If you remember that every villain is a hero, it’s often helpful to look at the whole story from the other guy’s point of view. Don’t just ask what the villain wants. Ask what the villain needs. Look for an arc so that he can change as well.

That doesn’t mean you ultimately have to split your time evenly between hero and villain. You almost certainly shouldn’t. But knowing what that guy’s movie would be can help you find the best story overall.