Character change is a big part of why we come to stories, and a good antagonist, opposing the hero or heroine, forces change. So it's worth digging more deeply into your antagonist to be sure you're getting everything you can out of him, her, or it.
I've listed 20 questions that I hope will be helpful. I focused on the idea of a villain, but it shouldn't be too difficult to interpolate these questions to whatever antagonist you have in your story.
- Is your villain active? Does he or she plan and execute work that impedes, distresses, and discourages your protagonist?
- Does your villain take advantage of the flaws and weaknesses of your hero/heroine?
- Does your villain go as far as possible in creating havoc and damage? Is the harm done extreme and is the villain willing to sacrifice to make it as difficult as possible for the hero/heroine?
- Is the villain vigilant and attentive? Does the villain react to what the protagonist does?
- Are his or her responses timely, apt, and punishing?
- Is the villain humanized? Are there enough dimensions so that readers might suffer the discomfort of identifying with him or her?
- Does the villain change as the story progresses – not in terms of focus, but in terms of improved ability, knowledge, and judgment?
- Does the villain surprise? Are some of the choices unexpected while being reasonable?
- Is the villain in some way a reflection of the protagonist? Would the hero or heroine feel uneasy about some of the things they have in common with the villain?
- Is the villain powerfully motivated? Are there reasons for his or her goal and do those reasons push him or her hard?
- Does the villain at some point offer the hero or heroine a choice? (This might include an opportunity for them to join forces.)
- Is the villain powerful?
- Is he or she at least the equal of the protagonist in terms of intelligence, resources, and options?
- Are the villains plans and goals clear? Where possible, is what the villain wants tangible?
- Is the villain's potential for causing harm illustrated early enough in the story to make readers dread his or her success?
- In addition to keeping the protagonist from his or her goal, to the actions of the villain undermine the hero or heroine's sense of identity or self?
- Is there a time where it's clear that there's more to the villain than meets the eye? Does a backstory add texture and depth?
- Are there important scenes where secrets about the villain are revealed?
- Is the villain fresh? Is there something so distinctive and interesting about him or her that his or her mere presence makes the story special?
- Do the actions and intentions of the villain support the story's theme? (Often, the villain stands in for the status quo or society's rules or values.)
That means you may find this set of questions in this series to be the most fun to work with. And that will increase the delight of your readers.