Here’s a one-dimensional story. A villain is doing something bad for the sake of being evil, and the hero defeats the villain just because the hero is always good. Boring, right? Here’s how to make it more interesting.
Your villain has his own goal and your hero has his own problems. Neither one cares about the other. The only reason the hero and villain get tangles up is because the villain threatens to wreck something the hero wants.
Your hero typically has two reasons to act. Without any reason to act, then your hero is just doing things for the sake of the plot, which makes for an unrealistic story. Your hero can’t just have one reason to act, but two to make the hero’s motivation stronger and more believable.
In “Star Wars,” Luke acts because of his fascination with Princess Leia and with his connection to his father through Obi0wan, who teaches him about the Force. If Luke wasn’t intrigued by the hologram of Princess Leia, he would have no reason to rescue her. If he didn’t have Obi-wan around to teach him about the Force, he would have no reason to leave his planet in the first place.
In “Die Hard,” Bruce Willis is trying to fight back against terrorists who are trying to kill him. Plus he wants to get back and save his wife. Get rid of his wife and Bruce Willis has no need to keep fighting the terrorists. Get rid of the terrorists and there’s nothing stopping him from getting back with his wife.
In “Gran Torino,” Clint Eastwood plays an old man who befriends an Asian family next door. He’s motivated to act not just out of friendship, but also because he wants to make up for the fact that he shot a kid in the face during the Korean War and also because he’s dying and doesn’t have much longer to live anyway. Just give him one reason alone to act and the story doesn’t feel as realistic.
In “WALL-E,” WALL-E acts because of his love for Eve. Then when he’s damaged, he needs to get back to Earth for spare parts. Take away his need for spare parts and he doesn’t need to get back to Earth. Take away Eve and he would never have left Earth in the first place.
In your own story, look for two ways to motivate your hero. More than two reasons might be fine, but two strong reasons make the hero’s motivation understandable so keep it simple. Your hero needs more than one reason to act to make it realistic, but three or more reasons might make it difficult to understand your hero’s motivation. Two good reasons is enough.
For yourself, what are your two reasons to write screenplays? If it’s just to make money, you’re doing it for the wrong reason.