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Subplots are secondary storylines that either support or contrast the theme of the main storyline. If your theme is a thesis that your protagonist will prove either right or wrong, then the subplots are the different sides of the argument. They're the pros and cons. Supporting the thesis through one storyline and contradicting it through another enriches the story and generates curiosity about how the protagonist will resolve his dilemma. What will he choose? How will it turn out? Subplots also help give us a better sense of character.
One of the subplots in the movie Moonstruck (written by John Patrick Shanley) involves the main character's parents. Loretta's mother discovers that her husband, whom she truly loves, is having an affair. This subplot mirrors and plays out the theme, which is true love is a risk worth taking. Her mother's story underscores Loretta's dilemma: Will she play it safe in a loveless marriage? Or will she follow her heart and embrace vulnerability, as her mother did?
Subplots are complete stories with a beginning, middle, and end. They have their own protagonists, usually a secondary character. The subplot protagonist often goes through a change, but it's a shift rather than a profound transformation. That kind of journey is reserved for the hero.
Your decisions about where and how to begin and end your subplots will affect the pace of your entire story. You may want to have the major subplot climax coincide with the A-story climax, or slightly before or after. Be careful not to dilute the A-story climax with a too-big subplot climax. Be sure to wrap up any loose subplot threads in the resolution.