Dashes (mdashes) and parentheses. Both of these are punctuation of the spotlight variety. I have seen them used correctly, incorrectly, and creatively. They are efficient tools to guide a reader along the path of voice and rhythm we've written into our stories. Once you know the "rules" you can effectively break them and call it style. But--how to use them?

Note--there are always exceptions, and there are many more ways to use them. I'm open to displaying every one of them in comments! But I'm not going to put them all in here.

Parentheses: I, personally, don't like them used in fiction unless it's a first-person narrative sort of thing; and for that reason I love them in non-fiction/memoir. Anything in parentheses is an aside, a little bit of extra information your reader might like to have, but is not necessary to the actual story being told. 

What do you set off with parentheses? Usually, a fragment, a date, even a full clause.
ex: My husband and I went out (date night!) that dark and stormy evening, completely oblivious to the chaos waiting just outside our door.

Without the bit in parentheses, the sentence could read quite dark and stormy, indeed. Adding (date night!) gives it levity, suggests the chaos coming is of the comedic kind. And see, while that works well in a non-fiction story, or a first person narrative, it wouldn't work as well when the character in question is not directly addressing her audience.

She and her husband went out (date night!) that dark and stormy evening, completely oblivious to the chaos waiting just outside their door.

See the difference? While the first is character intrusion, part of the voice of the story, the second is author intrusion.

Another way parentheses are often used (good for non-fiction; not so good for fiction) is an aside of information--like I just did! Ha! Didn't even mean to do that. Or like this:
ie: Jerome was born in a cave (March 19, 1809) in the Catskill Mountains.

In this sentence, the date of his birth is an added bit of information that doesn't really matter to the story. More on that in a moment. Now, on to...

Dashes (or mdashes*): This is the bolder cousin of the parentheses, and quite popular in fiction. I've used it quite a bit--overused, in fact, like right there. Bad me. I tend to overuse punctuation, and have decided that it's because I am Italian, and use my hands as much as my voice when I talk. In the absense of hand gestures, I use punctuation. Hey, it works for me! Doesn't always work for my editor.

But back to the mdash. While parentheses shine a soft, inobstrusive little spotlight on a detail, mdash shines one of those air-raid spotlights on a bit you want to make sure stands out. You can have a pair of them, like this:
Giada--darling of the debutante ball--got what was coming to her.
By highlighting, darling of the debutante ball, you spotlight the inconsistency, and thereby give the second half of that sentence more impact.

Or you can have one mdash:
Brilliant as all the night was, Tricia could not escape the fact that something was missing--Jordan.

Traditionally, one would use a colon instead of that mdash, and it would be perfectly acceptable; but the mdash gives it a bit more drama. It highlights the fact that Jordan is quite important, and perhaps heretofore unappreciated by Tricia. The mdash affords a bit more flair than the studious old colon. It is not a new thing, but it has gained in popularity in recent years. Personally, I'm in love with it.

One last way to use the mdash that I will cite here is like so:

Jerome was born in a cave--March 19, 1809--in the Catskill Mountains.

Why take away the parentheses and use the mdash? Because in this example, the date isn't an aside; it is an important piece of information pertaining to the story being told. Perhaps it is the same date another character was born, or something else quite important happened that day. Whatever it is, it is spotlighting a fact you want your reader to be aware of, not just offering an interesting but unimportant fact.

(*Little bit of triviia--did you know it is called the mdash (or emdash) because it is the width of an m? I had no idea. I thought it was an mdash because, without going into your settings and reformatting stuff, you have to type a letter after the double dashes to get it to make an mdash (one solid line.) I always use the M, and figured everyone else did too! Ah, the ego--because it's all about me, right? Ha! And see? I set this off in parentheses because it's an aside. I'm so clever.)

However you use the mdash or parentheses in your writing, know why they are used before you start getting creative with them. Learning to use your own judgement about such things is creating your own voice, and whether an editor is a hard-and-fast sort of rule person or not, voice trumps this sort of punctuation every time. Well...almost.