A directorial beat is an exchange of behaviours between characters in a script

Screenplay

A screenplay or script is a written work that is made especially for a film or television program. Screenplays can be original works or adaptations from existing pieces of writing. In them, the movement, actions, expression, and dialogues of the characters are also narrated...

. It usually takes the form of action-reaction. Each scene

Scene (film)

In TV and movies, a scene is generally thought of as the action in a single location and continuous time. Due to the ability to edit recorded visual works, it is typically much shorter than a stage play scene....

 beat by beat, with the characters progressing the action in this, the smallest element of story structure.

The directorial beat should not be confused with a screenplay beat, which indicates a short pause.