You know the cliche about there being two sides to every story? Cliches are a truth made banal by repetition - but still a truth. In a world where writers are encouraged to investigate their protagonist's character arc and emotional journey, it's easy to forget the antagonist. They may be the bad guy, but they're on a journey too.
Everyone's the hero of their own life story. You make your antagonist deeper, richer and more compelling as a character if you put some thought into their goals, their wants, their needs. The antagonist shouldn't simply be an obstacle for the hero to overcome. The more dimensionality you give your antagonist, the stronger they become.
Can't recall from whom I stole the following tip, but it's a good 'un: plot your second act from the antagonist's point of view. Good genre writers are wary of having a passive protagonist in their narrative. But too often in such stories the bad guy(s) are just as guilty of being passive, reacting to events rather than setting the agenda.
If the antagonist is always two steps ahead, it makes the hero's task that much harder - and that much more compelling. Need a good example? Try the film Se7en, where the detectives are always scrambling to catch up with an antagonist who's been planning events for more than a year. That gave the film an uncomfortable edge, but it was also bloody compelling.
Yesterday I was thinking about a new pilot script I'm developing. Didn't write a word, but I was happier at the end of the day than at the start. Why? Because I dug into the antagonist's POV. Suddenly a through-line emerged, pulling together disparate plot threads and weaving them into a story. Love it when that happens. Onwards!