I have an inordinate fondness for–some might say obsession with–intrusions.
Not physical ones. I don’t get into breaking down doors or smashing windows. I’m not talking B&E here. I mean written ones, like the one in the first sentence in this post. That clause in the parentheses is an intrusion. Why did I choose em dashes over parentheses?
To me, it feels more connected to the sentence it’s intruding on than it might if I worded it differently, so I set it off with ems. There’s a sense of “setting aside,” but not as far to the side as it could be. I can’t quantify it, I’m sorry to admit. It’s a sensibility, one of those things one develops over time, with experience. Another editor might feel the opposite of this, and prefer to use ems for things that “feel” like larger departures and parens for ones that “feel” less so.
But I digress. The main issue here turns out to be mechanics. Specifically, commas.
If your digression/intrusion requires a comma afterward, you’ll have to use parentheses. It’s simply Not Done, putting a comma after an em dash when it’s signifying an intrusive thought.
She picked up the soda (ordered an hour or so before, and now quite warm), the croissant (also an hour old, and now rather cold), and her phone, and left the diner.
If she was only picking up one thing, you could use ems:
She picked up the soda–ordered an hour or so before, and now quite warm–and left the diner.
NB: However, you can put a comma after an em when the dash signifies cut-off speech, like this:
“Didn’t even leave a tip. What a cheap bi–,” he muttered, censoring himself before anyone could overhear.
That comma is not only acceptable, it’s required. Declarative statement within quotes: put a comma before the closing quote if you’re using a dialogue tag.
But, as usual, I digress. (A rather necessary digression, because I know if I don’t cover that someone out there will bring it up and claim I omitted it even though it’s not exactly on-topic for intrusions.)
(Notice how the period goes inside the closing paren.)
(There. It happened again.)
So, back to the topic. Intrusions are part and parcel of fiction writing, in my experience, but are very rare elsewhere. If you’re writing fiction, whether there’s dialogue or not, you’ll likely find a use for an intrusion. (If you hate them, that’s fine. I’m not prone to inserting them where they were absent, but I’m all about making sure the ones you use are correctly punctuated.) If your intrusion requires a comma afterward, it needs to be set aside with parentheses. If no comma is needed, you can use em dashes.
I’ll leave the question of which feels more “set aside” for you to contemplate. Sensibilities vary.