I recently came across an observation about the roles of protagonist and antagonist in a story. The concept that the protagonist of a story is the one who changes and the antagonist is the instrument of that change. For example, in The Adventures of Huckleberry Finn, we know that Huck is the protagonist, but it could be argued that Jim is the antagonist. Jim challenges Huck’s assumptions in the world, instigating a vital change in his character.
Likewise, in the film The Shawshank Redemption, one can argue that Red (the character played by Morgan Freeman) is the protagonist—he is the narrator of the story and the viewpoint character—while Andy Dufresne (the character played by Tim Robbins) is the antagonist. Andy is the instigator in the transformation in Red’s character. Red begins the story without hope. A broken man. By the end, his transformation could not have occurred without Andy.
Likewise, in Shakespeare’s Othello, Iago serves as the antagonist—this time a deadly one—but his hatred for the Moor is not what defines him as such. Instead, it is the fact that he does not change but inspires change in Othello himself that defines Iago as the antagonist.
Like most observations about literature, I do not mean to say this is an absolute truth in every story—it most certainly is not—but it helped me refine the roles of my own antagonists in stories. I hope it helps you do the same.