So you’ve got your protagonist’s story planned all the way through. Beginning to end. You know what they want and what they need. That character arc is firmly in place.
What about your antagonist?
Have you put as much effort into developing their story? Do you explain why they’re doing this? What do they seek to gain from their actions?
A lot of the time, the bad guy is the more interesting character, so why wouldn’t you make just as much of an effort on fleshing them out?
The character we identify as the villain should see themselves as the hero of their story, with your protagonist the one standing in their way of achieving their goal.
Maybe there’s a previously-existing connection between the two, which can be gradually revealed as the story progresses.
How often has a writer explained the “why” behind the antagonist with a casual “Because they’re bad”? Readers and audiences want a little more depth than that.
This isn’t saying you need to come up with an extensive backstory about their past and what led them down this path. A few lines of dialogue can be just the thing to provide the reason why they’re doing this.
You’ve already spent a lot of time developing your hero’s journey. It only makes sense to do the same for the villain.