One of the first things I learned when doing Script Frenzy was that writing a script was much more structured in a technical sense than a novel or a short story. As I mentioned under Dialog, there are different elements that make up a script, and each are arranged and formatted a different way to make them easily stand out to a reader. A good script writing program, like Celtx or Final Draft, does as much of this for you as possible, letting you pick what element you want easily so that you can concentrate on what you need to write.
There are five elements that are frequently used in screenplays. The first is scene headings, which quickly convey where the scene must be shot and how. These are formatted in all capitals, flush against the left margin, and usually shaded in a gray background or something similar. The scene heading starts with ‘INT – ‘ for interior shot or ‘EXT – ‘ for exterior shot, and finishes with ‘ – DAY’ or ‘ – NIGHT’ – which can affect the lighting, and is also used to help track the chronology in the script. In between, you quickly describe the setting in a short phrase.
Action elements are used to describe what the characters are doing or what is happening in the scene aside from dialog. They’re also used to provide more detail on the setting after a scene heading. They start flush against the left margin and end ragged against the right margin, and are typed in easy to read sentences and paragraphs. Character names and props are CAPITALIZED when we first meet them, and important sound effects are always capitalized when they happen.
Character elements tell you who’s speaking. The character names are capitalized and left-aligned to a tab stop very close to the center of the page, so that most short names will appear more or less centered. You try to avoid using really long names here to make the dialog quicker to read.