There are many aspects of a story that deserve special attention during the editing phase, but correcting dialogue and the mechanics surrounding it requires extra time and focus. This is crucial if one wants to produce a story that creates and sustains "the fictional dream" that John Gardner maintains is critical to the success of a piece of fiction.
The foremost concern with dialogue is whether it's believable. Even though the work is a piece of fiction, readers need some degree of familiarity to latch onto while reading. A writer must ask herself whether her characters' lines reflect realistic speech patterns and if their diction and register are appropriate for the situation. Advice offered almost universally about revising dialogue is to read the lines aloud. This allows the writer to use her skilled ear to detect issues such as awkward phrasing, overly stiff speech, or even implausible fluidity in the conversation--no one in real life speaks without ever hesitating, faltering, misspeaking, or occasionally cutting off the other person in the conversation, so the characters in a story should also stumble or interrupt, at least a bit.
Other important aspects of dialogue in a story, counterintuitive as they may seem, are what goes unsaid and what should go unsaid. This is actually a two-fold concern. For one thing, writers should avoid using dialogue to further the plot of the story or to explain ideas and actions that are the purview of the narrator. It wouldn't be normal for people talking in real life to describe the setting to one another or provide back-story out loud to give context to their interaction; in an actual real-world exchange, those things would be known to the involved parties, and thus wouldn't make it into their conversation. Fictional conversations should adhere to the same logic, so characters' conversations should be interactions, not platforms for explication.
"Beats" are the other crucial unspoken aspect of scenes in a story. Beats are essentially body language or physical actions that accompany or take the place of speaker attributions. They precede, punctuate, or follow characters' utterances. Attributions such as "she said" or "he asked" are important to anchor readers and help them keep track of which statements are coming out of the mouths of which characters. However, they need not--and should not--accompany every line of dialogue in a story.
This is where beats become useful. If a character is upset, it is vague and uninspired to express that as follows: " 'I can't take it anymore,' he said sadly." Everyone has his own individual idea of what it might sound like for a person to utter something in a sad manner, so a writer who relies on this sort of phrasing is shortchanging her readers. Using a nebulous adverb to modify a speaker attribution depletes some of the richness there was to the fictional dream the author had been creating; the forming of that dream relies on detail, and using an unspecific term does nothing to enhance or sustain the fictional dream. Instead, the writer should dive deeper, "up the ante," so to speak, and use a beat to convey that her character is sad or upset, beyond the message revealed by the character's line. For example, the reader would get the idea that the speaker is sad--rather than angry, frustrated, or bored--if the author were to write, " 'I just can't take it anymore,' he said, then lowered his head, doing his best to avoid making eye contact with anyone at the table."
To intensify the impact of this line and the accompanying actions, the writer could employ the principles necessary to create realistic speech in concert with these non-spoken tactics. Perhaps the rewrite would go something like this: " 'I…I just,' he lowered his head. 'I just can't take it anymore.' His eyes bored into the ground, protecting him from making eye contact with anyone at the table." This version supplies readers with visuals that help depict the character's emotional state, using clear and relatable actions to enrich the scene in a concrete way. Additionally, the initial stumbling of the speaker lends believability to the line and amplifies the emotion behind it. It avoids distracting the reader with unnecessary or artless explanation, nor is it so vague that it prevents the reader from being able to relate to the scene.
Paying attention to the believability of the dialogue and staging scenes within a piece of fiction with intention are critical to the success of a story. Good dialogue moves stories along in a way that is indispensable and is entirely different from the work of narration. To get the greatest returns out of scenes with dialogue, writers must have the finesse to make the unreal real without losing the magic that separates fiction from that which is matter-of-fact.