on CMOS Shop Talk:
Interruptions happen all the time in real life. People talk over each other and past each other; words collide and overlap. Sometimes an action or a thought rather than a person intrudes, causing a speaker to stop abruptly or, less dramatically, to trail off midsentence.
The conventions in fiction for signaling such interruptions on the page depend on the conventions that allow for dialogue in the first place, so let’s start there.