An Em Dash Cuts Dialogue Short. An Ellipsis Lets It Trail.
Two punctuation marks. Same real estate at the end of a dialogue line. Opposite effects. “You don’t understand, I never meant to—” That em dash is a door slamming. The speaker got cut off. Maybe someone interrupted. Maybe the thought collided with something louder. Either way, the sentence didn’t end. It was killed mid-breath. The…