Dialogue is the central feature of any screenplay. The prose passages are kept brief as the director will be interpreting and executing them, but the dialogue is the heart of the characters and story.
Though the conversations may be similar to other literary forms, the way that it is put on the page is dramatically different. This does not just come from the paging structures that come with screenwriting, but also the way that punctuation is used in the dialogue. When you are screenwriting, there are uses of punctuation in dialogue portions that are different than in any other creative situation.
The first thing you have to remember is that this is dialogue that is meant to be spoken out loud and not just read. Everything that is put in is done so that when it is spoken out loud it is understand clearly. Do not over punctuate, and make sure that all use is warranted for a person who is trying to convey something through their speech.
The big thing that tends to be used in script dialogue is the hyphen. The hyphen is used for either a sudden break or for extra emphasis. This is very useful for indicating when someone’s dialogue was interrupted by another person’s speech or action. It is also used in different formatting positions.
A space followed by two hyphens and another space is where you may put a description. A space followed by a single hyphen and a space is what you use in headings to separate different elements.
The ellipsis is also used commonly in screenplays. This is often used to connect piece of dialogue where there is a pause. People often pause periodically in regular conversation, and the ellipsis is a good thing to indicate this regular space.
Always use brackets or parenthesis to indicate an action that happens in the middle of dialogue. This is perfect is someone does something while speaking or during a momentary pause while talking
You have to remember that people tend not to talk in large sentences. Spoken word is much different than that used in literature, so your punctuation must reflect this brevity and simplicity. Keep the use of complex literary punctuation, like semicolons, down to a minimum. If you notice excessive punctuation, it may be an indication that you need to do some revision.