Action runs from left to right margin, the full width of the text on the page, the same as the Scene Heading. Be sure to use the word wrap function of your script writing software, to make editing and rewrites easy. Text is single-spaced and in mixed case.
When you introduce a speaking character for the first time, you should put the name in all caps.
Script writing software intuitively formats the spacing and text between different paragraph styles for you as you type. All you worry about is your story!
The ACTION or Description sets the scene, describes the setting, and allows you to introduce your characters and set the stage for your story. Action is written in REAL TIME.
Every moment in a screenplay takes place NOW. Use the active voice (a window slams shut) not the passive voice (a window is slammed shut).
Always write in PRESENT TIME, not the past. (There are rare exceptions to this; for example, John Milius' The Wind and the Lion had description in past tense like a novel, but then, he also directed the film.)
Keep your paragraphs short... don't let them go on and on over 4 or 5 lines. The reader may scan long action paragraphs without really reading them.
FADE IN: EXT. KEY WEST MARINA - DAWN - ESTABLISHING Sailboats, yachts, and cabin cruisers all bob up and down in the warm blue water. INT. MASTER SUITE - MORNING Expensive designer sheets and comforter covers the nude, shapely body of drop dead gorgeous JULIE COOPER, 25. Sunlight filters through portholes over the muscled, tan body of FRANKIE CAMPISI, 38. He pulls the comforter down and begins kissing Julie's naked body.
The reader begins to form an idea about the setting and the action taking place. We know we're on a boat, two characters have been introduced to us, we have some idea about their physical appearance. And we have a clue to their relationship.
Avoid a compulsion to write camera angles and shots. If you must emphasize some shot, write it on a single line. Angles and shots are the domain of the director an will likely be added in the Shooting Script.
Expensive designer sheets and comforter covers the nude, shapely body of drop dead gorgeous JULIE COOPER, 25. Sunlight filters through portholes over the muscled, tan body of FRANKIE CAMPISI, 38. He pulls the comforter down and grins at Julie's naked body. Suddenly, Frankie recoils. There's a devil tattoo on her shoulder that he's never seen before.