Something rather less demanding, for a change: to consistently differentiate between pauses (ellipses) & interruptions (em dashes) is not only good orthographic behavior, but also increases narrative options.
If you have a good editor, you should keep him or her warm and cozy and not make the same mistake a trillion times over until it builds up bad vibes and fumes and stuff and turns your editor at the stroke of midnight into something really terrible and nasty. And if you don’t have an editor, you need to know this anyway.
Now this is pure and simple:
- If your character’s dialog or inner monologue is interrupted by any event whatsoever, including being distracted or startled by something, use an em dash. (If you don’t know how to produce an em dash on your keyboard, you can use two en dashes as long as you stick to one method or the other.)
- If your character is running out of steam, trailing off, or making conspicuous pauses during a dialog or inner monologue, use ellipses. (You might come across ellipses with spacing between the dots. Don’t. All kinds of mischief will ensue, and they make for funny linebreaks.)
So how would that look? Meet Mr. Pynchon, one of the masters of pauses and disruptions, in this quotation from Gravity’s Rainbow:
Slothrop is listening to faraway peripatetic tuba and clarinet being joined in on now by trombone and tenor sax, trying to pick up a tune … and to the bursts of laughter from the soldiers and girls … sounds like a party down there … maybe even some stag dames … “Say, why don’t we, uh … what was your—” Närrisch, leather scarecrow, trying to ignore Slothrop’s behavior, has decided to dismantle his firebomb: he uncorks the vodka and waves it under his nose before taking a belt.1
I took this example because it shows another advantage of doing things right. If you’ve managed to establish the difference between trailing off and being interrupted through the consistent use of these tell-tale signs of advanced typography, you can take your time to explain what it was that disrupted or startled your character without running the risk that your readers will be losing out on the effect.