Q. Hi! Is it ever appropriate to follow an em dash with a period if it’s the terminus of the sentence? Thanks!
A. You could try it, but there are probably better options. When an em dash marks a midsentence interruption of one speaker by another, as in quoted dialogue, it’s best not to use any additional punctuation (see CMOS 6.87):
“I thought I might—”
“Might what?”
A period after the dash would suggest that the sentence had come to an end rather than having been interrupted. But even if the speaker breaks off and then resumes speaking without a paragraph break, a period wouldn’t be necessary:
“I thought I might— Oh, it’s no use.”
not
“I thought I might—. Oh, it’s no use.”
Either option could work (and note the space after the dash in the first one), but the period in the second example makes the break seem less sudden.