And I'm now reluctant to use dashes because of their casual ambiguity.

That's why I like them!

Must dash.

Terry, yes, I see what you mean.

If they don't know which to use, as so often, it's as much about them not being quite clear what they're trying to do, isn't it. If they were, then which punctuation to use would be obvious.

casual ambiguity.

Nothing wrong with ambiguity, rightly used. It's central to how free indirect style works, for a start...

I think the thing about dashes is that the true function of a dash is to indicate that the normal structure of the sentence is interrupted.

That has its function - and it's a very important fuction - in normal, written prose.

But it's even more common in informal prose, and in speaking - in speaking especially.

And although fiction is written prose, it always has one foot in how voices work (see Bakhtin); it's always working with how people actually articulate their consciousness in speech (even when it's not dialogue).

So the dash that expresses how the normal sense and structure of a sentence is also used to indicate the vocal/oral pause - the pauses we do in speech, where the - stuff - things change - because they do. Because fiction's like that. Isn't it?