It may be considered rude to interrupt someone, but doing it reciprocally is somehow okay. In a strange, almost beautifully coordinated way.
Sometimes you just have to do it—for example, when you’re talking to somebody that just won’t stop talking. To borrow a term from Peter Schickele, I’m talking about a talky-talk kind of person. At that point you can either slide back into the conversation with a shiny change of subject (when they’re taking a breath, maybe) or you can let them go on about—
Sorry, what were you saying? I’m having trouble focusing today.
You may have noticed that people who like to talk a lot have adapted to being interrupted. They’ll listen to you carefully and they truly, genuinely care about what you have to say. But at some point, when you take a breath or finish a sentence, they’ll pay you back in kind. That talky-talk will finish what they were saying earlier. (And you’re standing there thinking they had nothing else to say. You need to pay more attention! You should all go for some coffee…)
The em dash usually represents a planned interruption of a line of reasoning—or, in dialog, one person interrupting another. It can replace the elipsis, or even a pair of parentheses. It’s a wonderful piece of punctuation, but it’s easy to overuse and can rapidly wear out its welcome.
Sorry. Back to mister talky-talk:
(There are plenty of miz talky-talks out there as well. Many, many of them. My wife and I are perceptive people, and we both know the other needs us to be talkative.)
It’s too easy for you to to judge mister talky-talk just plain rude. But if you have a look at talky-talk’s friends and family, you may notice that they have the same schtick he does—interrupting, bringing the conversation back to the original point, and all that talking. Talk talk talk talk. Even if it takes a while, most everybody gets their say. And the conversation may even be the better because of it; the “interruptions” end up working their way into other threads of the conversation, feeding off each other in a nearly musical way.
Writing about a conversation like this would be a perfect opportunity for you to overuse em dashes; and even if you used them properly, your page would still be infested with them. You might consider breaking things up by rewriting a little (just enough to be able to use parentheses, or other punctuation).
The semicolon and, in particular, the full colon, tend to be associated with convoluted sentence structure, and I’ll cover what I consider good usage in a future article. (See what I did, there?) But if you use a variety of punctuation, and if you don’t overuse the same sentence structures in your writing, it should all work well. In the end, you just have to remember one thing: Always know your audience. Some people hate being interrupted, no matter the context, and some readers hate em dashes. Hate hate hate hate. (And these same readers—and editors—often hate parentheses, and any highfalutin’ punctuation other than periods and colons. But you can’t please everyone.)