A slug line tells the reader where we are, what we are looking at. Slug lines are in caps, followed by a double space to the next unit, which usually is an action line. An action line describes who is in the scene and what is happening. Beginning writers overwrite action lines. Often, in the action line, a character name is in caps the first time it is mentioned, not in caps thereafter.
Some writers use caps or noncaps throughout.
Here is an example of a good action sequence , from a work in progress by Mike Snyder. Character names are in caps, in the center of the page. Only a single space separates character names from dialogue. Dialogue is indented, both left and right. A double space follows dialogue and the next unit, whether it is a character name or an action line.