Your antagonist should be as well fleshed out as your protagonist
How many great books have you read in which the antagonist is poorly defined, or worse, a caricature of a villain? I can’t think of one. In the best stories, the antagonist is as memorable and engaging as the protagonist.
Think of Darth Vader, Severus Snape, Voldemort. What about Sweeney Todd, The Phantom (of the Opera), Magneto, and Harley Quinn? What is it about these villains that make us love to hate them, or even empathize with them?
Creating a villain who oozes evil isn’t enough to keep readers engaged
Especially not for the length of an entire novel, or, heaven forbid, a series. Cardboard cut outs become tired and annoying in a hurry. They’re not interesting to read and, more importantly, they don’t engage the reader’s emotions, and emotion is what keeps eyes glued to the page.
Give your antagonists nuance. Make them complex, fully fleshed out people with as much backstory as your protagonist. Know them as well as you know your hero. Try some of the following to make them real people who engage your readers and keep them turning the page.
Give villains a backstory readers can identify with
Create sympathy for your antagonist. In the television series Once Upon a Time, the evil queen, Regina, starts out as a kind, caring person. Her backstory shows a young Snow White inadvertently betraying her love for a stable hand, and Regina’s mother killing him. We understand what has hardened her heart and created her hatred of Snow. However much we hate Regina, a tiny part of us sympathizes with her.
Write from the antagonist’s point of view
Let your readers into your villain’s inner world. Remember, each character in your story is the protagonist of their own story, including the antagonist. They often believe their actions are good and just. Thanos, from the Marvel Universe, is motivated by a desire to cure poverty and suffering. He just happens to believe the way to do that is to wipe out half of all living beings in the galaxy. We can both simultaneously hate this character and understand that he is operating from what he perceives as a motivation for good.
Show your villain’s inner conflict
Very few villains are single-minded automatons bent on destruction. Perhaps the most iconic conflicted villain of all time was Judas, who betrayed Jesus to the Pharisees. He loved Jesus, and yet believed him to be dangerous. He handed him over to be crucified, then hanged himself, unable to live with his choice.
Darth Vader is another famously conflicted antagonist who, in the end, finds redemption when he saves his son Luke from the evil Emperor. The Star Wars franchise continues this theme with Kylo Ren, who, also conflicted, takes the opposite path, murdering his father.
Create similarities between your villain and hero
Give them similar backstories. What led one to good and the other to evil? In Megamind, both Megamind and Metroman were sent to earth as infants. They received very different welcomes and upbringings, and this set their life paths.
Maybe your antagonist and protagonist share certain character traits. In Pride and Prejudice, Lizzie and Darcy share an overabundance of both faults. Perhaps both hero and villain, for their own reasons, are chasing the same goal. Classically, Cinderella and her stepmother are both trying to impress the prince.
Give your antagonist a story arc
You can give glimpses into the villain’s past to show the arc that brought them to their role. Alternatively, a story arc concurrent with that of the protagonist’s journey can be either redemptive or a further descent into evil. Redemption can be complete or muddied by lingering negative traits. You could have a villain move toward redemption, then step away, completing their original agenda. There are so many variations on this theme.
Darth Vader starts life as Anakin Skywalker, who enters Jedi training with good intentions. He descends into the role of villain and ultimately is redeemed by his love for Luke.
However you go about it, you must make your antagonist a fully fleshed out character, as complex and compelling as your protagonist. The worst fate for villains is for readers to feel emotionally untouched by them. Love them or hate them, it doesn’t matter, as long as readers viscerally connect. Your job as their creator is to make that happen.