What does (BEAT) mean?
Some people will tell you it's a way to get an actor to pause for something important. It tells the actor when to take the breath. It's a dramatic equivalent to OMG!!!!;) Or something.
Others will tell you it simply indicates a pause.
Most writers go through a (BEAT) phase. (BEAT) starts littering the script wherever the writer wants to indicate something significant, or worse -- weighty. And the writer convinces himself that he makes his script weigh more by doing so.
Then there's the added benefit of telling the actors how to act. If you put that (BEAT) in there, well, they need to stop and savor your brilliance.
Not surprisingly, actors bristle at scripts full of (BEAT), just as they do at scripts full of "smiles knowingly", "claps his back reassuringly", or (subtly, with rising anger). Actors are not puppets, and you are not a puppeteer.
And actors usually know what a beat is.
A beat is simply a unit of drama. It's a moment that moves the story forward. It's not a moment that represents something, or elaborates, or even 'weighs' anything. It is movement. It is forward motion.
In virtually every drama virtually all of the time, the beat happens through the actor. How?
They change their intention. They react to a new circumstance. They try to effect a change in another character.
These are all things that happen first in a character's. So what does (BEAT) mean to an actor? It tells her she needs to consider how her intention changes. It tells him that a shift has occurred in his character's psychology.
Sometimes this even happens without a pause for emphasis.
(BEAT) used correctly engages the actor's training. They look for interesting possibilities in the character. They uncover connections that need to be brought out. They consider your lines in a different way. You're telling them to make a choice, a gamble, a deeper read.
And when they read more deeply and just come up with (ponderous pause)? You've bored them or worse -- you've lost their faith in the script.