The basics, what on earth is an Em Dash?It is a dash the size of the letter M (would you believe that's how it got its name?). Microsoft word will create an em dash for you when you put two dashes together as such --. Do not put in a space before or after the em dash.
I will look at a FEW common uses here (there are a couple more):
The em dash can be used to signal a break in thought or tone:Take a look at how F. Scott Fitzgerald used it in The Great Gatsby: "I lived at West Egg, the--well, the less fashionable of the two, though this is a most superficial tag to express the bizarre and not a little sinister contrast between them." On that note, and completely unrelated to this post, check out Hemingway's criticism of F. Scott Fitzgerald's Tender is the Night--it will make you feel better about your critique partners feedback.
Okay lets get back to it, here is another example of the em dash used where there is a break in thought: The trees stretched high into the sky--Jane wondered if she'd ever seen trees so tall.
If the quote itself is interrupted the em dash will go inside the quote:"Jane run, before they--" a shrill scream shattered the otherwise silent night. It was too late.
It can be used to break the quote with narrative as such:"Jane run"--he pointed behind her--"they're almost over the wall."
It can be used where you have an abrupt, startling appositive or where you want to emphasize the appositive:First lets look at an appositive without the em dash, rather it is is punctuated with commas:
Jane, a skittish teen who faints at the sight of blood, stood on wobbly legs as the red liquid pooled around her.
Now with em dash emphasis:
Jane--a skittish teen who faints at the sight of blood--stood on wobbly legs as the red liquid pooled around her.
To set off parenthetical fragments (or sentences) especially where it relates to the main thought:Jane ran--desperate legs sprinting as fast as they could, her arms swinging for momentum--targeting the back-seat of the waiting truck.
To summarise the previous series:Guns, explosives, knives--those were Jane's favourite weapons.
Use of the em dash:So how often should the Em dash be used? This is probably personal preference, however, I think because the em dash can be a powerful tool of emphasis you don't want to be using it all the time--especially because it can be quite jarring to read when used too frequently.
How about you, how often do you like to use the em dash?