Just the basic rules for the dash. It is the third one that many people don’t always remember.
There are three basic reasons to use a dash:
1. broken sentence structure (This is where the interruption fits.)
2. full sentence dropped into another full sentence
…I want to read to you from — can you hear me? — your deposition on May 5.
…I want to read to you from — this is page 15 — your deposition on May 5.
3. summary dash
…They insert the needle directing into the brain — a scary thought.
…We left early that morning — John and I.
…That is what I was talking about — that he is not ever on time.