How to punctuate dialogue correctly

Dialogue punctuation is not a matter of taste. It follows a small set of fixed rules, and a reader registers a mistake even when they cannot name what is wrong. A misplaced comma after a quotation makes a sentence look amateurish before the reader has even judged your prose. The rules take an afternoon to learn properly. Most writers never sit down to do that, so a bad habit calcifies scene after scene until it is load-bearing.

The comma before a dialogue tag

When a line of dialogue is followed by a tag such as she said or he asked, the punctuation inside the closing quotation mark is a comma, not a period. "I'm not doing that," she said. Swap that comma for a period and the tag becomes its own sentence, which is an error rather than a stylistic choice, because said no longer has a subject doing anything else in the sentence.

Questions and exclamations work a little differently. The question mark or exclamation point takes the place of the comma when the line itself is a question or an exclamation: "Are you serious?" she asked. You do not add a comma after the question mark, and asked stays lowercase, because the tag is still grammatically part of the same sentence as the quotation.

Action beats are not dialogue tags

A dialogue tag names how something was said, using a verb of speech: said, asked, whispered, muttered. An action beat describes what a character does, and it needs a period, not a comma, because it is a separate sentence. "I'm not doing that." She crossed her arms. That period is doing real work. Write it as "I'm not doing that," she crossed her arms and you have turned crossing your arms into a way of speaking, which is not what happened, and a reader feels that wrongness even without being able to name it.

This is the single most common dialogue punctuation mistake in manuscripts I read: treating an action beat as if it were a tag. Fix it by asking whether the verb after the quotation actually describes speech. If it does not, it gets a period and starts a new sentence.

Capitalizing what follows the quotation

The comma-versus-period choice above decides how the next word is capitalized too. After a comma, a dialogue tag stays lowercase unless it opens with a proper noun: "I'm not doing that," Maria said, not "I'm not doing that," Maria Said. After a period, because the action beat is its own sentence, it starts with a capital letter regardless of what the verb is. Writers who get the comma and period right often still stumble here, capitalizing a tag that follows a comma out of habit from writing regular prose.

New speaker, new paragraph

Every time the speaker changes, start a new paragraph, even if the previous line was one word. This is a formatting convention rather than a punctuation rule, but breaking it damages readability more than almost any comma error does. A page where two speakers share a paragraph reads as confused, no matter how sharp the individual lines are, because the reader has to work out who is talking from context instead of from the page's shape.

Interrupted and trailing dialogue

Dialogue cut off by another character or an event ends in an em dash inside the quotation marks. Dialogue that trails off on its own, rather than being interrupted, ends in an ellipsis instead. Both marks sit inside the closing quotation mark, because the interruption or the trailing-off is part of what the character said, not part of the sentence describing it.

Dialogue inside dialogue

When a character quotes someone else inside their own line, the nested quotation uses single quotation marks: "She told me, 'Don't wait up,' and then she left." The outer double quotation marks still carry all the rules above. The single quotation marks around the nested quote don't get their own comma or period logic; they simply wrap the quoted words wherever they fall.

"I'm not doing that," she said.              comma before a tag
"Are you serious?" she asked.                question mark replaces the comma
"I'm not doing that." She crossed her arms.  period before an action beat
"I told you not to —" he started.           em dash for an interruption
"I just thought maybe…" she said.           ellipsis for trailing off

Punctuation you can automate, and punctuation you cannot

Some of this is mechanical enough to hand off. Curly quotation marks instead of straight ones are a formatting detail, not a grammatical choice, and Chaptered's editor has a smart quotes preference that runs a typography extension to convert straight quotes to curly ones as you type, so you don't have to fix them by hand later. The comma-versus-period decision above it is not something software can make for you. It depends on whether the words after the quotation describe speech or an action, and that judgment is still the writer's to make, one line at a time.