Dialogue and Narrative
Grammar is essential to good storytelling; it keeps the reader from getting lost. When writing narrative good grammar is essential. But when your characters speak they talk like human beings. People don't speak in semicolons and neither should your characters.
Robert Harris wrote a trilogy about the great legal orator Cicero: Imperium, Lustrum, Dictator. In these stories, whether speaking in private or conducting a public oration, Cicero does not speak with semicolons in the dialogue.
Natural speech is a key element in creating an empathic character. Your editor may get stuck with the fine points of grammar within dialogue, but your readers want a character to speak in flow, just the way real people do.
An editor sparked the idea for this post with a comment about the lack of semicolons in a character's speech. My reply was the title of this post: Characters don't speak in semicolons.
Simple tricks to dialogue
As a writer, you can enliven your dialogue by writing in natural speech flow. The trick is to use punctuation and possibly break some grammar rules.
Your readers will understand. They don't speak in semicolons either.
On the other hand, you'll want to make sure your dialogue is punctuated correctly for interruptions, breaks, and attributions.
Editor Jodie Renner provides useful guidelines in her article for Kill Zone.
A. Ellipsis (…) or Dash (—)?
Dialogue is the Spice of Character Building
Dialogue is one of the strongest ways to get your readers emotionally involved with a character.
When I wrote the introductory scene for Cassiodorus in Ravenna: A Mosaic where he speaks in long, convoluted sentences and does not get to the point, one of my fellow writers said, "Tell me he dies before the book ends." Now that's an emotional response. He was sorry to hear that Cassiodorus lived on into his nineties, well outside the time frame of the story.
Your character may speak in monosyllabic words or long phrases. Either way, make the dialogue reflect your character and how he or she interacts with the other personae in your story.