May I Interrupt? The Dash
The dash is a handy device, informal and essentially playful, telling you that you're about to take off on a different tack but still in some way connected with the present course — only you have to remember that the dash is there, and either put a second dash at the end of the notion to let readers know that he's back on course, or else end the sentence, as here, with a period. — Lewis Thomas, M.D., from The Medusa and the Snail (1979)
Ah, the “playful” dash. As Mr. Thomas writes, a dash provides a window for some informality in writing, allowing the writer to introduce an abrupt change in thought or tone. Look at this sentence:
The odometer just reached thirty thousand miles, so it's time to call the garage for — oops! I just passed the street where we were supposed to turn.
The dash tells readers that a sudden idea interrupted the speaker's original thought.
Use a dash to give emphasis to something that's come before. Look at this sentence:
Elizabeth spent many hours planning what she would pack in the van — the van that she had rented for two weeks.
Another time a dash may be used is in defining or giving more information about something in a sentence. Read this sentence:
Margaret knew that when she finally arrived at her sorority house, she would be warmly greeted by her sisters — Bea, Kwila, and Arlene.
The last example could also be punctuated by using parentheses or a colon in place of the dash. You might have written the same sentence this way:
Margaret knew that when she finally arrived at her sorority house, she would be warmly greeted by her sisters (Bea, Kwila, and Arlene).
or this way:
Margaret knew that when she finally arrived at her sorority house, she would be warmly greeted by her sisters: Bea, Kwila, and Arlene.
You can see that punctuating the sentence with colons is much stuffier than using a dash or parentheses. Generally speaking, save the colon for formal writing.