There are two sides to this theme in The Return of the King. First, the language the characters use says a great deal about who they are, where they come from, and what they represent. Then, there's communication on the whole, and in this book, communicating is all about storytelling. In Middle-earth, stories have a power all their own. And in telling these stories in the guise of an academic history (with those fancy appendices), Tolkien lends a realness to a fantasy world that you can't find anywhere else.
Sure, Tolkien uses speech and dialogue to indicate a character's social place (just look at Sam and Frodo's different ways of speaking). But he also uses it to distinguish between different levels of cultural development. Ghân-buri-Ghân talks like a stereotypical Wild Man, while the men of Gondor speak something like Elvish as their native tongue, which shows their high level of cultural achievement.
The fact that Pippin speaks so directly while Denethor speaks suspiciously is proof that the way a person talks is a surefire indicator of what kind of person they are in The Return of the King.