If you're a writer, you've probably heard that piece of advice before, from other writers, industry folk, or one of any number of the dozens of screenwriting how-to books, manuals, blogs and treatises that are out there. It's perhaps the most obvious and, dare I say, pedestrian thing you can tell a writer.
The impulse behind the saying is valid. It speaks to the need to write characters and situations that feel real and dimensional, and theoretically an iron-clad way to do this is to write about people or scenarios you have lived and/or witnessed. Your script can't be phony if it is transcribed from real life. But just because it's real doesn't mean it's interesting.
After all, don't we go to the movies to escape real life? To be captivated and transported? Cinema (and arguably all forms of narrative expression) is about high-stakes conflict, regardless of genre. As writers, we need to craft premises and characters to fulfill that fundamental expectation, and real life may not always be the ticket to create reel life.
"What you know" may not necessarily be cinematic, and in that case, writing what you know isn't only limiting, it's bad advice. If what you know doesn't sizzle, it's not going to gain traction and get sold. It's that simple. Hollywood is the niche market to end all niche markets. Your audience is creative executives, agents and assistants with cynical dispositions, short attention spans and a staggering lack of free time. You couldn't ask for a rougher crowd, so you have to make your script count, and that starts with a premise, a logline that pops and engages and hooks. Anything less than that is not going to cut it.
So write what you know only if you believe that it's high-stakes and cinematic, and, just as importantly, if it's something that you want to write about. Don't think that just because you haven't lived the life of a globetrotting CIA operative that you can't write that. That's what research is for! Your challenge then is to plug your real-life emotions and experiences into that premise and those characters that are foreign to you. Writing is hard enough without limiting your possibilities from the outset. Pick subject matter that will sustain you through writer's block and the patches of self-doubt.
Write what you know...as long as it's cinematic and something you actually want to write about. That may be a wordier piece of advice, but it's right.