There is this Yiddish proverb I learned in my first writing workshop; "A wise man hears one word and understands two." This notion inspires the utility of words in dialogue.
First, the dialogue that takes place in fiction does not accurately reflect real and informal day-to-day conversations. Countless writing schools and sources will challenge students to attempt writing out a conversation as they hear it, whether it be in a coffee shop or a zoom meeting. Obviously, normal conversation is jarred with filler words and does not consistently flow or offer clear emphasis on a plot.
When it comes to dialogue in fiction the words need to be somewhat useful in detailing the relationship between characters and who they are. It can emphasize the traits of the character speaking or of the person the character is speaking to. There is also room for more dignified exposition the kind that is not overexplanatory, uninvited, or unnatural.
In Frank Herbert's Dune, the Baron and his suite are introduced. In this exchange, the reader learns of the characters' nature and motives. Herbert could have simplified the exchange into nearly a summarized account, something like this:
In Duke Leto's letter, he blatantly denies the Baron's title to drive the nail into his refusal for a meeting, which is considered rude due to the Baron's authority. Leto's justification is in the Baron's own untrustworthiness. In response, the Baron's Mentat, Piter, mocks Leto's position as the new ruler of Arrakis, a desert planet that will be strong-armed by the emperor.
Notice how this moment is translated into an effective scene. See how each line of dialogue contributes to the character and the plot. Here is what is actually written in the novel:
"Well?" the Baron demanded "The fool answered us, Baron!" "Whenever did an Atreides refuse the opportunity for a gesture?" the Baron asked. "Well, what does he say?" "He's most uncouth, Baron. Addresses you as 'Harkonnen'- no 'Sire et Cher Cousin', no title, nothing." "It's a good name," the Baron growled, and his voice betrayed his impatience. "What does dear Leto say?" "He says: 'Your offer of a meeting is refused. I have ofttimes met your treachery and this all men know." "And?" the Baron asked. "He says: 'The art of kanly still has admirers in the Empire.' He signs it: 'Duke Leto of Arrakis.'" Piter began to laugh. "Of Arrakis! oh my! This is almost too rich!"
I really like to think of dialogue in fiction as a silent discussion between the reader and the characters. The reader witnesses the conversation, the gestures, reactions and responses, the threads of internal thought, all forms of indirect or unspoken dialogue. What makes 'unspoken' dialogue challenging for a writer is not knowing where it may have the most impact. A creative writer may want to deploy as many details as possible in order to create a cinematic scene. As valuable as such details may be, it is important to know when details will be useful for a reader rather than a viewer.
Having recently indulged in Dune, I will include another example from the novel. Herbert does an excellent job at balancing scenes of dialogue with character actions and internal thought. In this scene, the 'unspoken dialogue' communicates to the reader the exhausted state of a seasoned warrior, a servant of the Atreides family. This moment spares the reader of a bland explanation of his experience and describes how the character carries himself in a conversation:
Thufir Hawat slipped into the training room of Castle Caladan, closed the door softly. He stood there a moment, feeling old and tired and storm-leathered. His left leg ached where it had been slashed once in service of the Old Duke. Three generations of them now, he thought.
Following this, the reader gets a glimpse of Paul's perspective of Hawat:
Paul looked up at the grizzled old man who stopped at a corner of the table. Hawat's eyes were two pools of alertness in a dark and deeply seamed face. "I heard you coming down the hall," Paul said. "And I heard you open the door." "The sounds I make could be imitated." "I'd know the difference."
And not long after, the reader also gets to see how Hawat sees himself:
A training table remained, and a fencing mirror with its crystal prism quiescent, the target dummy beside it patched and padded, looking like an ancient foot soldier maimed and battered in the wars. There stand I, Hawat thought. "Thufir, what're you thinking?" Paul asked.
You never really get to know someone until you speak to them and see how they behave. It is just that way for characters. Dialogue is not just a simple conversation. It is an exchange of gestures and feelings that can elevate the story's delivery.