When a drastic change occurs, it occurs in a relatively small and isolated population.
- Ernst Mayr
I was born in Evanston, IL: population 75,543. Grew up in Kenosha, WI: population 96,240. Went to school in Milwaukee: population 573,358. And now I'm a husband and father of three, living and working in Los Angeles: population 3,849,378. According to the most recent estimates from the United States Census Bureau, I went from small to bigger to the second largest city in the country. But those are just numbers. When it comes to screenplay story, population stems from your main protagonist and the characters involved in his or her specific world.
As an infant in Evanston, my population was pretty simple: mom, dad, and me. But mom and dad divorced, and mom and I had moved to Kenosha before my third birthday. I lived there for 15 more years, and as a kid in a relatively Norman Rockwell community, the population of my life changed depending upon which story I was in at the time.
Wisconsin summers filled with dirt bikes, basketball, and kick-the-can were pages right out of Rob Reiner's Stand by Me. There was four of us: Bart, Brian, Clint, and myself. We were inseparable, a cohesive team, battling our adversaries: the old witch in the corner house who'd call the police every time we'd climb her evergreen, the bully who'd chase us out of Newman Park with a seven-iron and an endless supply of golf balls, and the entire staff at The Brompton School after we accidentally set their parking lot on fire. Our population was small: four 12-year-old boys. And we were, as Shakespeare said, truly a "band of brothers." But that's just one story - one population.
There are dozens of different populations in everyone's lives: coworkers, classmates, the yoga group at the YMCA. But you can't determine what the specific population of your screenplay is - and it must be specific - until you decide whose story it is first. Start by creating a complex and flawed protagonist who has a clear objective. Make sure this protagonist will learn, grow, or change in some way after the experience; this helps build a powerful character arc. Do your best to provide polarity in order to maximize the opportunity for conflict. But most importantly, keep things small.
In screenwriting, less really is more. There is a reason only six college friends go off to the cabin in the Horror film. The elite commando unit, regardless of genre, is always small and well contained: Aliens, The Seven Samurai, Predator, Saving Private Ryan. Every rom-com has best friends for each male and female counterpart.
I may have grown up in a city with 96,240 people, but the population of my many worlds was always small, always specific. Even today, living in Los Angeles, a city that's sure to break four million once all the census forms are tallied, the central population of my most important world is only five: my wife and kids.
So don't reinvent the wheel. Fill the population of your screenplay with interesting characters and use polarity to create opportunities for conflict, but always limit your population. It's a small world after all. So keep it that way.