Screenplays evolve essentially two ways: plot driven (often called High Concept), when a writer plugs original characters into a tailor-made plot, or Character Driven, in which the plot is born organically from the characters, usually an unforgettable main protagonist.
Character Driven Screenplays
With this approach, if you take away the character, there is no story, because it is the character (not the idea) that is the cornerstone of the screenplay. The character is the lifeline: the heart, the mind, and the soul of the entire script. Think Jeff Bridges as Bad Blake in Crazy Heart (2009), Kate Winslet as Hanna Schmitz in The Reader (2008), or Mickey Rourke as Randy in The Wrestler (2008).
Story Driven Screenplays
With this approach, often the title alone is enough to understand the entire story: Wedding Crashers (buddy comedy), RoboCop, (crime/action), How to Lose a Guy in 10 Days (rom-com). All successful films, but in each scenario, the characters are secondary to the story idea. Imagine Twister or Jaws or Speed with completely different characters. The story still works.
In Hollywood, the term High Concept refers to a movie that can easily be described by a succinctly stated premise, but also is considered easy to sell to a wide audience because it delivers an easy to grasp idea. Marketing, cross-promotional advertising, and merchandising is all part of the package. Back to the Future, Night at the Museum, Jurassic Park… Toys, T-shirts, amusement park rides. You get the idea.
Any genre can be High Concept. Supernatural/Comedy: Ghostbusters. Action/Adventure: Armageddon. Family Comedy: Home Alone. And it’s true that many studios pitch and develop High Concept movies almost entirely upon a premise rather than complex character study, but there’s nothing stopping you from doing both. If you’re going to go High Concept, make it original, and that starts with character. Nobody really cares about Vin Diesel’s Riddick, but there’s no doubt that Indiana Jones will live on past the iconic actor who played him.
There is no right or wrong way to give birth to your screenplay. If the story idea strikes, create characters that fit in the mold. But if it's the character that emerges, allow yourself to explore him or her. Put that character in different situations, different worlds, use character exercises to explore your character further, and most importantly, trust your character. A great character will lead you to a great story.