In every great story, you’re really telling one story using multiple characters. In every bad story, you wind up telling different stories with multiple characters. The end result in a bad story is that by telling so many unrelated stories, no single story stands out and the overall story becomes a mess.
In “Wonder Woman,” there’s an early scene by a British spy whose job is to observe what the Germans are doing. When he discovers they’re creating a deadly poison gas, he violates orders, steals the German chemist’s notebook detailing her plans, and steals an airplane that he uses to bomb the laboratory. He says he did this because he couldn’t stand still and let something horrible happen without trying to stop it.
Later when Wonder woman is visiting the trenches of World War One, she meets some refugees who tell her about their village being overrun by Germans. Like the British spy, Wonder Woman also refuses to let something awful go unchecked so she too charges into action to right the wrong.
This parallel story between the British spy and Wonder Woman strengthens the overall story because it shows how both of them refuse to let something bad happen if they have the power to change it.
Parallelism occurs in every great story. In “Casablanca,” the hero, Rick, is a cynical man who only looks out for himself. When he changes, he decides to help others. At the same time, his friend also makes the decision to help Rick and thus help fight against the Nazis too. Both characters change in similar ways, which strengthens the overall story.
What would happen if in “Casablanca” Rick decided to change and help others while his friend decided to find a woman to love? The two different stories wouldn’t support each other and so they would actually interfere with each other.
Think of every weak story and you’ll see story lines that don’t support each other. Watch “Suicide Squad” to see so many different stories that none of them support each other. One character, Deadshot, wants to get back with his daughter. Harley Quinn is in love with the Joker who keeps trying to rescue her. The later of the group wants to rescue his girlfriend who has turned into the Enchantress. These multiple stories don’t parallel one another very well and then you add in the stories of the other characters and they make even less sense. Is it any wonder that “Suicide Squad” turned into such a mess?
Every story needs multiple storylines and those storylines need to parallel one another. The more closely two different characters have similar types of goals, the stronger the overall story will be.