Think of the dash as indicating an interruption you want to call attention to. Other punctuation marks—commas and parentheses—serve similar purposes. Commas are more neutral, and parentheses usually enclose information that is extra and incidental.
If you want to interrupt your sentence with a phrase or clause, consider using a dash. Or if the sentence continues after the interruption, use a pair of dashes.
She was extraordinarily tall— the tallest woman I'd ever seen.
She walked in— the tallest woman I'd ever seen—and took a seat at the counter.
Introducing a restatement or explanation
Like a colon, a dash can be used to introduce an explanation or restatement in place of expressions such as that is, in other words, or namely. Begin the clause after the dash with a lowercase letter.
The reporter relentlessly pursued the woman— he was determined to get her to make a statement.
Although the colon and dash are frequently interchangeable in this function, the dash is less formal.
Dashes with commas
When you use dashes to set off interrupting elements in a sentence, omit commas.
She saw her sisters— all five of them— standing in front of the building.
NOT She saw her sisters— all five of them— , standing in front of the building.
Dashes with quotation marks
If a dash is not part of the quoted material, put it outside the quotation marks. Omit commas.
“He wants the money”— I paused for effect—“and he wants it now.”
A dash can be used to indicate unfinished dialogue. Put the dash within the quotation marks and omit commas or periods.
“ Help! Help! I can't seem to—” She fell to the ground, gasping for breath.