“8. Use a dash to set off an abrupt break or interruption, and to announce a long appositive or summary.”
A dash is a mark of separation, stronger than a comma, less formal than a colon, and more relaxed than parentheses.”
–The Elements of Style
I like “mark of separation”. It is reassuringly informal. Dashes, commas, colons, semicolons–for all their complex circuitry, all they really do is separate. They make your brain (or breath, if you’re reading aloud) pause.
This supports a view I’ve always liked about punctuation, that it’s our best of our ultimately futile attempts to put the nuances of speech into print. Have you ever written something to someone that offended them, but you know that if you had said it, they would have found it every bit as funny or empathetic as you?
Choose your pauses carefully.