On one hand, the term action is used to describe the main story of a script - meaning a sequence of events leading to a definite outcome. On the other, it means an exertion of energy, an effort to achieve some objective, involving the character’s thought, emotion, and will. In other words, action implies a purposeful pursuit of an objective.
And the bulk of the screenplay is the description of the actions and activities of the characters. The effective screenwriter thinks of the actions of the characters and how they should be seen by the audience. This is the heart of dramatic writing.
Action reveals character. It’s not so much what a character says; it’s what a character does that is important. The old adage “actions speak louder than words” is the general truth. What matters is not what happens; it is how the character reacts to what is happening.
You must get into the inner lives of your characters - their joys, torments, secret desires and aspirations, hidden fears. It is the juxtaposition of dialogue and action, very often mismatched, that gives us our clearest picture of the inner world of a character.