An element of literature and film I find a lot of fun is the power of two completely opposite characters being brought together by a common event or goal. Once there, they find themselves in a scenario where they want precisely the opposite things.
The two will not compromise; they cannot see eye to eye. If not for the unavoidable goal or event in front of them—be it organising the funeral of a parent, being caught in a natural disaster or involvement in a big project at work—the opposites would probably part ways.
Opposites abound in romance. You’ll see it whenever the enemies to lovers, opposites attract and grumpy-sunshine tropes are used, and often in the forced proximity and marriage of convenience tropes too. A unity of opposites is more powerful because it amps up conflict by placing seemingly insurmountable barriers at every turn.
Whether the relationships are romantic or not, the best stories always have a unity of opposites. Some of my favorite examples appear in family dramas and thrillers. Who can forget the pairing of Isobel Crawley and the Dowager Countess of Grantham when they’re thrust together upon Matthew Crawley’s unexpected inheritance of Downton Abbey. Neither will budge and both believe they are in the right. In every scene!
This device can also be used without limiting it to relationships that resolve well. Used with skill, it can help explore the irresistible forces at work behind the antagonist’s “crime” and create reader empathy by showing how the antagonist was unable to avoid ending up here.
This unity of opposites can bring a shared respect between the characters. Above all, it allows the writer to form the protagonist’s character through conflict in an interesting, and often fun, way.