That's the philosophy behind some writers, they will only write what they know. The thing about writing the kinds of thrillers I want to write is there is a lot of police/political/legal research that goes into the plots. In fact the research takes so long that it's probably best to make up the technicalities, if need be, and just keep on writing. But some writers just prefer to write what they know, and stay away from researching or making things up. So is it a good idea, to only limiting yourself to what you know, or is it better to just not let realism bother you and make up technicalities to serve the story, if need be?
We all know a lot more than we realize. And all the answers have held the issue up and looked at it from all conceivable angles. Screenwriters--all writers at heart really--are storytellers. Stories are not about events and details but desires and relationships. E-Motion is what sets Events into Motion.
A loves B and B leaves A for C. B now copes with the love and loss.
The gender of A, B, and C -- male, female, animal, object, idea, etc. varies as does time and setting.
However, the "classic love story" remains.
Coming into a movie, the audience has no clue about the world they are stepping into, but they "know" emotion. And that is the primary instrument of the writer as storyteller. Suspense, horror, passion, etc. all come from what is desired and the paths that lead to it or deny it.
Even before I became a father, I could imagine having a son or daughter. And it's funny how that works. I would see someone at McDonald's or the park playing with a kid and think what does it feel like to be the kid? And as the father? And you know, holding my sister's kid, it brings thoughts and associations into my head. Sometimes I think writers believe that imagination is in the details, when it's really about the feelings.
Now, having said that, there are lots of movies which do require research. In European languages, there are two ways of 'knowing'--knowing facts (saber, wissen, savoir) and "knowing about/acquainted" (conocer, kennen, connaitre). A good screenplay uses both forms of knowing. In the end, though, the audience is more able to relate to the 'feeling' side. Anyone who thinks "House" is how real doctors (even good ones) should approach diagnosis and treatment would be sadly dead. The same for many police and law shows. It's the characters and situations--not the legal details--that carry the story. You need to write to the general knowledge level of your audience unless its meant to enlighten.
You should only write what you can believe and feel. If you can do those, your storytelling will take the reader/viewer where ever your characters take them. If you can't convince me that you believe it or feel it's real, as a reader/viewer, the story/movie doesn't make the grade, no matter how clever or well researched. I think screenwriters would benefit from an acting class. Learning to step into another role, especially one different from yourself, helps you to step inside characters.
As the White Queen advised Alice in "Through the Looking Glass", she 'could believe six impossible things before breakfast' and encouraged Alice to do the same. It certainly is good advice for screenwriters. Can you believe your the father of twin girls? Believe that you can transfer your consciousness into an alien species and what it would be like? Or imagine finding an small alien in your garden shed? -- What would it feel like as a single father raising them? What would you experience with the new senses having a tail? How would you hide the fact from your mom being only seven? You might have to sit on the floor to get reacquainted with (re-'know') an old perspective--it's a different world being 4 ft (~120 cm) again.
But realize that the aphorism applies to only one piece of screenwriting--the story. The other two pieces are format and structure. Poor dialogue, Poor pacing, poor development have less to do with 'knowing' and more with 'crafting'.