Sage
Calling Occupants of Interplanetary Craft
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Obviously, there are ways that an author can change up the way they expect the sentence to be read. If “and then” is meant to be the end of the sentence or cut off or trailed off, and, “He died,” is meant to be read as its own complete sentence, punctuating it will look different from what I am about to say.
But if the entire spoken sentence, said without pause, is, “He stood there for three days, and then, he died,” the sentence containing the speaker’s action is punctuated with em-dashes on the outside of the dialogue quotation.
Chicago Manual of Style
CMOS shop talk
But there could be different ways that the OP could get around this, and because this question comes up all the time, but I rarely see an example in books, I suspect authors often revise their interrupted dialogue to avoid this situation prior to publication anyway.
But if the entire spoken sentence, said without pause, is, “He stood there for three days, and then, he died,” the sentence containing the speaker’s action is punctuated with em-dashes on the outside of the dialogue quotation.
Chicago Manual of Style
If you want to leave out the speech tag, that same convention would require either em dashes or periods, because now the narrative interruption has lost its immediate connection to the spoken dialogue:
“So up there”—Joe pointed at the window—“that was you waving at me?”
CMOS shop talk
I’m failing to see how the OP’s example differs from these two.Normally, dashes belong to the interrupted speaker. But when the narrator intervenes in the middle of a speech to describe an interrupting action or movement, the dashes are better placed outside the quotation marks. Do this even if the speaker’s words are also interrupted:
“Don’t you dare”—Cassandra paused for a moment to glare at Ralph—“interrupt me.”
But there could be different ways that the OP could get around this, and because this question comes up all the time, but I rarely see an example in books, I suspect authors often revise their interrupted dialogue to avoid this situation prior to publication anyway.