As writers, we have to consider the relationship between spoken words and the characters. Diction, simply meaning words, is a commonly used term that is rarely taken to its literary extreme. Often, it’s used to create vivid imagery; no easier example would be poorly written gore that relies on the use of repetition, shocking descriptions, and evocation of the reader’s disgust.

This type of diction and its relationship with the reader is one of straightforward command. The words tell the reader the scene, and thus the reader can picture it in their head.

“Rainbow trembled, her hair standing on edge from fear as her heart ripped through bone and sinews with its panicked beating. Those hollow eyes, grin peeling flesh back, revealed evil in her gnawing teeth.”

Little else is said here, and as we will see, the increased detail can become ineffective. An expansion of detail is needed to elicit an effect of the reader. Subtlety in diction can be a lot more powerful. Take, for example, the work of 14^th Century English poet Geoffrey Chaucer. In his compilation of stories, The Canterbury Tales, one of the tales include an exchange of dialogue between a witch that has coerced a knight into marrying her and the Queen of the land. She addresses the queen with “you” before turning around and speaking to the knight with “thee, thy, and thou.”

If you study another language like French and Spanish--Latin rooted languages--you would understand the concept of a second person singular which is used informally, and a second person plural that is used formally. In Chaucer’s time, English also had this distinction. The “you” was applied as the second person formal plural, while “thee” was reserved as the second person singular, and was used for informal, familiar conversations.

Because the story involving the witch and the knight—the Wife of Bath’s Tale—is one of female control in marriage and romantic relationships, the significance of the distinction between “you” and “thee” is astoundingly critical. The witch talks up to the queen, using the “you” tense as a show of respect, while her use of “thee” to the knight is, at best, one of familiarity, and at worst, condescending and lacking of respect. Here, two different words placed in the same space are able to convey social status and reveal how the witch sees her relationship with the knight she has manipulated and gained control over.

Sadly, we are unable to replicate this exact example in modern writing, but the rule still holds that deeper reflection on our connection and understanding of the words we use is crucial to developing deeper, more revelatory diction that can increase the literary quality of our writing.

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If I understand your piece correctly, you are comparing dialog vs description.
Dialog and description if done well are useful tools for any writer. It's like comparing apples to oranges.
Depending on your style and the situation you can mix either one at will.

“Rainbow trembled, her hair standing on edge from fear as her heart ripped through bone and sinews with its panicked beating. Those hollow eyes, grin peeling flesh back, revealed evil in her gnawing teeth.”

Little else is said here, and as we will see, the increased detail can become ineffective. An expansion of detail is needed to elicit an effect of the reader. Subtlety in diction can be a lot more powerful...

Show us how diction could have made this description more powerful then go on talk about diction in dialog.
I wanted to see how this could be improved.

Sadly, we are unable to replicate this exact example in modern writing, but the rule still holds that deeper reflection on our connection and understanding of the words we use is crucial to developing deeper, more revelatory diction that can increase the literary quality of our writing.

True but people didn't stop making ways to identify themselves through language. That's a part of culture.
By studying how people talk and seeing how they chose their words you can understand how to use diction in dialog to make your characters more realistic.