I'll just bet that you've heard or seen or somehow have become convinced that sound should be capped when it appears in your narrative. My, my, my ... it's so much harder to unlearn bad habits than it is to learn a thing correctly in the first place.
Once your screenplay has been purchased, there will be production meetings, and a shooting script will be typed up and distributed to your production team (This won't be the final shooting script, however, since that one is compiled after the film is completed).
The SOUND TECH will take a marking pen and mark everything in the script that pertains to sound. The Location Manager will mark the script for those particular needs. Wardrobe, casting, transportation, special effects, all of those people will be marking their script in highlighter pen for what they, specifically, need to do. Sometimes a crew member will confess that they have never read the entire script; they just cull out the parts that have to do with their job.
Once in a while I have to chuckle at the interpretation of a new writer's idea of "capping for sound". One writer capped "He BLINKED". Maybe I just don't get around enough, but I've never known a noisy blinker.
After a movie is filmed, it will go to into post production where the geniuses there will begin matching sound to action. One of the first bits of sound they add is the subtle rustle of fabric as characters simply walk. Another is adding natural sound (ambiance) in what appears, onscreen, to be a silent scene. If no sound is required in a particular scene, the camera shoots the necessary footage, then it's shown on a post production screen. A couple of people will stand in front of a mic and breath in and out, in a very normal way. The sound is recorded and "layered" over the scene to make a "natural" background, otherwise it would sound "dead" onscreen.
I keep seeing scripts that have the sound capped and the only thing I can figure is that the sound is critical to the scene and the writer doesn't want the Sound Tech to overlook it in the script. But, at this stage of your writing career, it's best not to try to determine what is critical to the Sound Tech and what isn't. You just tell the story. If you cap a sound incorrectly (i.e., "blinked"), it can only make you look like an amateur or, worse yet, look foolish. If you don't cap any sound you can't possibly make a mistake.
RULE: NEVER CAP SOUND IN NARRATIVE
"Okay," you say. "I've got it. I'm never to cap sound in my screenplay. What next?"
Not so fast, Impatient Writer. I didn't say that at all. I didn't say you never cap sound in your script. I said you never cap sound in narrative.
If a sound is truly critical to a scene (i.e., "An o.c. woman screams from the next apartment building."), then go ahead and underline screams, but do such things sparingly, and only if you're convinced it is such a vital piece of information that you have to make certain its importance is not overlooked. By the way, "o.c." means off-camera, but we'll get to that elsewhere on this website, at another time.
