Separating an interruption -- a phrase that breaks with the uniformity of a sentence -- from the rest of the sentence is almost always done with the use of appropriate punctuation. Although there might seem to be a mere handful of punctuation marks, each mark or set of marks can be used in a variety of ways to meet a variety of needs. When introducing an interruption to a sentence, three forms of punctuation, in particular, are most versatile and helpful.
Parentheses appear in a sentence in order to interject something important, but somewhat tangential, to its meaning. For example: "John's ex-girlfriends (Cindy is a chef and Toni is a painter) still send him love letters." Here, the material within the parentheses communicates important information about the identity of the women to which the sentence refers, but it is not absolutely essential to the sentence's meaning. In less formal contexts, parentheses may also be used to insert an aside to the reader or listener. For example: "I saw it with my own eyes (you know, I never lie), but I still can't believe it."
In prose seeking to reproduce the rhythms of natural speech, dashes might end a sentence in order to show that the speaker has been interrupted. For example: "Carla began to say, 'I am going on vacation with -- ' when the door slammed." Dashes can also take the place of parenthetical asides. For example: "Sarah headed for the door -- already late for work -- when she left her sack lunch behind."
Commas with Appositives
An appositive is a word or phrase similar in meaning to another noun or noun phrase in the same sentence. Like an adjective or adverb, the appositive works to modify the word or phrase to which it adheres. Commas are often used to surround appositives that contain information nonessential to the subjects they modify. For example: "Marilyn Monroe, born in California, was a famous Hollywood starlet." Here, the fact that Monroe was born in California is ancillary information that briefly interrupts the real purpose of the sentence, which is to communicate that she was a famous performer.
Commas with Quotations
Sometimes, a quotation is interrupted in the middle of a sentence in order to identify a speaker or to recognize an action taking place while the quote is being said or read aloud. In these cases, commas are used after and before each sentence fragment. For example: "She was a beautiful girl," Gary said, "but she was terribly rude."
Commas and Added Detail
A pair of commas used in the middle of a sentence can set aside words, phrases and clauses that, like ancillary appositives, contain details that aren't entirely essential to the sentence's meaning. For example: "Sundays, which are always a little depressing, are the only days I don't play baseball." Also, "The special effects were fantastic. The acting, however, was weak."