Final answer:
Use one dash to indicate that a speaker's speech is interrupted but then continues in a conversation. Dashes emphasize and clarify content and represent pauses or changes in direction in writing.
Step-by-step explanation:
Use one dash when the speech is interrupted in a conversation, but the speaker continues his thought. Dashes are versatile punctuation marks that can serve various purposes in writing. They are typically used to set off or emphasize content within a sentence, create a pause or indicate a change of direction, and place more emphasis on enclosed information than parentheses or commas.
For example, during dialogue, if a character's speech is interrupted by an action or another character, and then the character resumes speaking, a single dash can represent the interruption. Consider this dialogue: "I was just about to say—" John paused as the phone rang "—that I can't come." The dash after "say" indicates the interruption, and the one after "rang" shows where the speech resumes.
Dashes are also used to set off appositive phrases that add explanatory or clarifying information to nouns, such as in the example, "The cousins—Tina, Todd, and Sam—arrived at the party together." This usage of dashes helps to avoid confusion that might occur with additional commas.
In conclusion, when aiming to emphasize or set off a phrase in writing, particularly to represent an interruption in speech that continues, a single dash is utilized, while parentheses serve to de-emphasize or enclose nonessential information.