Why do screenwriters tell people to 'write what you know'?

[Kholi]

Great responses here.

Would also say that what you know doesn't necessarily have to be serious, it can also be fun and light-hearted.

[kylevant]

And Stephen Crane never served in the armed forces or saw combat--

http://en.wikipedia.org/wiki/The_Red_Badge_of_Courage

But Oliver Stone did, and that shows; even if only slightly, just enough.

[alex whitmer]

Writing 'what you know' doesn't need to be someting that happened to you. It can be something that happened to someone close to you, a neighbor, or even something more distant, but affected you in a way to inspire a story. This IS different than starting out with a raw idea of say, a heist if you have never experienced it, nor know anyone who has.

I don't agree that you should write what you know in your early writng days, as a rule. An inexperienced writer may find it more difficult to be objective when writing from personal experience, and insisit certain scenes and dialogue be in the story, even though they mess with the overall cohesivenesss of a cinematic interpretation. Writing something that has been done to death could likely be easier, so I for one do not adhere to the 'write what you know' perspective.

[novicewriter]

I think that's sound advice. If you don't write what you know, you're just making some stabs in the dark. I couldn't write about what I don't know anything about. But I know what you mean. I have to chuckle when I watch police shows. You never see a police officer read someone's rights as they're escorting the arrested away, or holding a gun next to their face instead of in a down position. I could go on. Of course it's not realistic, but it's more "dramatic" that way. The gun next to the face gives a nice close up for the camera, etc. On TV shows and movies, they even have experts on set. But you have to give the audience what they expect; what's real is often what's boring to watch. I'd stick with the advice, write what you know.