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?
How to Intrude with Style
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. (This could be considered a style choice. Some writers love ems and parentheses. Others don’t. It’s their style, their voice. And this is in parentheses because “style” isn’t the topic of this blog post, but this is definitely related to the topic.)
"Grammar Girl" Sez...
Mignon Fogarty, better known as “Grammar Girl,” explains that em dashes lend more oomph than parentheses to what they set aside. I concur. It’s that sense of connection to what surrounds the intrusion. If “added oomph” works for you, then by all means, use that as your measure. To read her thoughts on the topic, just search for “Grammar Girl em dash” and Bob’s your uncle. Or, you could click here.
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.
You may well be tempted to 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.
Em Dashes and Commas Don't Mix
Please don’t. It looks awful. But more important than that, it’s against the accepted punctuation conventions and rules. Em dashes and commas do not mix.
Change the tag to a beat so you don’t need a comma. Like this:
“Didn’t even leave a tip. What a cheap bi—” He bit off the word and chewed on its sourness as he shot daggers in her direction.
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.)
Intrusions Are Inevitable in Fiction
So, back to the topic. Intrusions are part and parcel of fiction writing, in my experience, but are very rare elsewhere. If you’re primarily a writer of news or academic articles, you won’t have much reason to use them at all; the expected and accepted register of such writing precludes the use of parentheticals. (For journals, the use of parentheses is reserved for in-line citations.) 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.
Parentheses Lack the "OOMPH" Factor
Em dashes signify additional “oomph” to the intrusion. Parentheses are for additional information that’s nice to have, but not vital. Ask yourself how important the intrusion is, and you’ll be halfway to choosing the better punctuation.