The em dash is a versatile punctuation mark which may replace a comma, colon, or parentheses in order to introduce extra emphasis. Em dashes often set off details that are noteworthy, unexpected, or shocking. In other words, they emphasize details that attract the reader's attention. As such, overuse of the em dash diminishes its effects. Utilize the em dash only when it is truly necessary, and try to limit yourself to two or three per synopsis.
- Better: Keisuke fell in love with Marina—his brother's wife.
- Satoshi—despite failing the previous eight tests—was sure that he would ace the next one.
- Suboptimal: Keisuke fell in love with Marina, his brother's wife.
- Satoshi, despite failing the previous eight tests, was sure that he would ace the next one.
An em dash should be used to set off a noun or a list of nouns which appears at the start of a sentence and is immediately followed by a pronoun that introduces the main clause.
- Fear, anger, and sorrow—these emotions clouded his judgment.
If you want Google Docs to automatically convert -- to —, go to Tools > Preferences and add -- under Replace and — under With. Alternatively, you may type an em dash on Windows by holding Alt and typing 0151. On Mac, press Shift+Option+Hyphen (-).