Single dashes are used to comment on, summarize, or explain what precedes:
Suspenseful, dark, and fast-paced—what more could you ask for in a movie?
The movie was the best of its kind—that is, suspenseful, dark, and fast-paced.
Paired dashes are used to parenthetically enclose elements of a sentence:
His marathon training was intense—if not grueling and downright obsessive—but in the end it paid off.
For clarity, restrict your use of dashes to one single dash or one paired dash in a sentence. Sometimes, this will require varying punctuation depending on your intended emphasis:
For best results, use a baking apple—say, Honey Crisps or Pink Ladies (that is, avoid apples that break down and turn to mush)—and blind bake your crust.
For best results, use a baking apple (say, Honey Crisps or Pink Ladies)—that is, avoid apples that break down and turn to mush—and blind bake your crust.