Dramatica co-creator Melanie Anne Phillips prepends a forward to one of her many insightful articles in this post on Protagonist v Antagonist | Dramaticapedia:
Now in reading this through today, I realize that doesn’t sound much like the way most writers go about creating their characters. In fact, the usual approach is to start with a protagonist and antagonist in mind, then populate the story with supporting characters to fill out the conflicts and the logistics of the battle over the goal.
This is, in fact, the approach I instinctively take and the one I follow when working with other writers. To me, the Protagonist and Antagonist of a story stand out as the most easily identifiable character in a story. One is for the Story Goal; the other works to prevent it.
Of course, identifying the Goal of a story is not always easy and different techniques exist to navigate this process. But Melanie explains it in a way that is both simple and complex at the same time:
In our own minds, we survey our environment and consider whether or not we could improve things by taking action to change them. The struggle between the Protagonist and Antagonist represents this inner argument: is it better to leave things the way they are or to try and rearrange them?
The Protagonist represents Initiative; the Antagonist Reticence. Follow that and your story finds purpose.