Because we see it so frequently, we tend to think of the dash as being synonymous with an interruption. We will be more accurate if we think of the dash as marking a break in the sentence structure, that is, indicating a sentence that got started and just didn’t get finished — for whatever reason.
Specifically, let’s look at this construction:
…Q The car that was in the left lane — was it the one that you feel caused this accident?
…Q This photograph I am holding — was this the one you took?
Note that, in the second part of the sentence, we have a pronoun that refers back to the noun we were talking about. The word it refers back to car. The word this refers back to photograph.
What is “The car…left lane” in the sentence? What is “This photograph…holding” in the sentence? Well, it is really nothing yet, and it doesn’t get to be anything because that first part of the sentence does not get finished. It does not really turn into a sentence.
So what is correct punctuation? A dash.
Rule: Use a dash for broken sentence structure, for a sentence that starts and does not get finished.
So often we see a comma in place of the dash in a transcript. Is the comma okay? Could it be correct? No, not really. We have no rule that says a comma shows a break in sentence structure. That is what we have a dash for.