Shakespearean plays begin with the chorus telling us what we are about to see. They don’t tell us the whole plot but rather provide a context for the events that are about to follow.
That is the purpose of the theme. It provides the audience with a context through which to understand the plot.
Our novel or screenplay is about something. If it’s just a series of events that lead to some vague shift for the protagonist, the audience would scratch their heads and wonder “What the hell was the point of that?”
The dramatic question or theme keeps our story on track.
It is the personal made universal, meaning it is something personal to the hero while at the same time being something we can all relate to. “The truth will set you free,” is a theme. “Crime doesn’t pay,” is another theme.
At the beginning of Goodfellas, we watch a teenage Henry Hill learning the ropes from the mob. When his father discovers that he is skipping school and has fallen in with a bad crowd, he beats him mercilessly. The message is clear. Stay away from crime and go to school. It is a nowhere life. The mob then beats the mailman so that he will no longer deliver Henry’s truant notices, and Henry can skip school without consequence. The theme falls into place. If we are going to show how crime doesn’t pay, we must show the opposing argument. For Henry, crime initially provides him with a sense of power, belonging, and excitement. Through the story the plot is going to lead us to the consequence of this lifestyle.
Themes are time-honored, universal truths. They are the reason we return to stories again and again. It contains the universal laws that govern evolution. We can discover them for ourselves by tapping into the primal drive of our protagonist and being curious about how these desires play out.