When used at the end of a sentence in scene description, there’s a measure of interchangeability (I’ll tackle that in another post). But when used at the end of a sentence in dialogue, things are very clear cut.
What’s the difference?
Three dots (an ellipsis) at the end of a sentence are used to indicate that a speaker has trailed off. For example:
But if my fingerprints are on the knife, then that means...
Jacqueline stares at her hands, the realization sinking in.
You’re the Sleepwalk Killer!
Two dashes (technically hyphens) at the end of a sentence are used to indicate that a speaker has been interrupted. For example:
Follow me to the kitchen. I want to show you my new knife collec--
Not a chance!
I made cinnamon buns.
The double hyphens can also come after a completed word, in the middle of a sentence, to indicate an interruption. I could have put the double hyphens after the word “knife” in the sentence above, but then that may have given the impression that the sentence had ended there.
If I didn’t want to write a partial word, then this rearranged sentence could work: “I have a new knife collection I want to show–”
Notice how there’s no space between the double dashes and the last word being interrupted? I feel it gives a better sense of an interruption that way.
A Note About Single Hyphens
Some screenwriters (including some pros) use single hyphens to indicate an interruption or a pause. I guess the idea is that the single hyphen takes the place of a longer em dash (—) that you would normally see in anything written with a proportional font instead of a fixed-width font like Courier — basically anything other than a script.
But we’re talking about a script. So the practice of using a single hyphen (for this purpose) is kinda weird to me, as it is to many other readers. Do you want to use something that may bump certain readers, or do you want to use something that wouldn’t bother a single reader?
Having said that, give me a script where the writing is otherwise fantastic, and I won’t even notice the issue.