I received a request for a post on how to use an ellipsis (plural: ellipses); this is it.
The tl;dr version: Use an ellipsis (…) to indicate either a dramatic pause within a sentence or a thought trailing off.
The Chicago Manual of Style makes a distinction between an ellipsis and what it calls suspension points. Technically, an ellipsis indicates an omission from a quote, and you’re unlikely to use that when writing fiction, although you may use it when writing blog posts, etc. Suspension points and an ellipsis look exactly alike: three dots in a row. As you may guess from the name, suspension points are used to indicate suspended or trailing-off thought, which likely will come up in fiction.
I’m using the term ellipsis to cover both, though, because, as far as I can tell, no one outside Professional Editor-dom (and not everyone in it) uses the term suspension points, and I’m writing these posts to eliminate confusion. Most of what I’ll cover here deals with ellipses in their “suspension points” aspect.
“This could be perfect, but…”
This is an example of correct ellipsis usage, where the speaker trails off rather than completing the thought.
My mind is being taken over by Prompts… And I love it!
In the above example, the ellipsis correctly shows a dramatic pause. However, the word and should not be capitalized (and neither should prompts, although that, as scientists would say, is outside the scope of this project), because it is a continuation of the same compound sentence that started before the ellipsis, not a new sentence. (A “dramatic pause” at the end of a sentence is best indicted with something other than an ellipsis. Save the end-of-sentence ellipses for trailing off.)
One common issue I see with using ellipses is not knowing whether to start a new sentence and capitalize the next word. The solution is easy: capitalize the next word or not as you would if the ellipsis weren’t there: “Hold on… You said this would be easy, dammit.”
Another issue is punctuation after an ellipsis. If a sentence, minus the ellipsis, would end in a question mark, add a question mark after the ellipsis. The same goes for exclamation points. (“What the…? You, get out of…!”) Do not add a period after thoughts trailing off.
In dialogue where there’s an ellipsis before the tag, add a comma after the ellipsis, just as if the ellipsis weren’t there: “The last time I tried this…,” she said.
 One more thing… I forgot to mention spacing. This one’s easy: Do not run the ellipsis into the next word/beginning of the next sentence. Include a space after the ellipsis, just as you would after a word or sentence.