Jody's style of speech is a bit different from the residents of Eatonville and especially from the country folk at Green Cove Springs. All the dialogue in the book is in dialect, but Hurston slightly modifies that dialect to show Jody's different style of speech.
One of the ways the reader sees this is through Jody's smooth, clever turns of phrase. Once he gets to Green Cove Springs, he starts to try to convince the people already living there that they ought to build an all-Black town. He then uses this rather funny idiom: "You cannot have no town without some land to build it on. Y'all ain't got enough here to cuss a cat on without gittin' yo mouf full of hair." This phrase ("so small you can't cuss a cat without getting your mouth full of hair") was common throughout the south in the 1930s.
Still, it is quite a descriptive phrase, so Starks shows his flair for dramatic oratory with this idiom. It also is simply a good way to describe that the surrounding land is small. Instead of a narrator simply saying that there was little land to build on, Hurston has a character describe that fact to the reader through a succinct and funny statement, much more enjoyable to read. It helps to characterize both Starks and the new town.
And Joe's dialect is distinct: he is a little closer to standard English, using "cannot" instead of "can't" and has fewer distinctive pronunciations. In doing so, Joe is trying to seem professional by emulating white styles of speaking, to assert a sort of power over the people living where he wants to build a town. This statement quickly builds background for both Joe and the new town, on several levels.
Hurston varies the use of dialect between different characters. In addition, Janie's thoughts are not in dialect while her speech is. Both of these contrasts help the author to build characters and their relationships and to show the differences between characters. The following is an example from a conversation between Janie and Nanny. The quotations are from Nanny, and Hurston supplies Janie's thoughts in between:
"What Ah seen just now is plenty for me, honey, Ah don't want no trashy nigger, no breath-and-britches, lak Johnny Taylor usin' yo' body to wipe his foots on."
Nanny's words made Janie's kiss across the gatepost seem like a manure pile after rain.
"Look at me, Janie. Don't set dere wid yo' head hung down. Look at yo' ole grandma!"
Nanny's dialect here is quite thick, especially in comparison to Janie's. Nanny uses "d" where other speakers, especially Janie, use "th" (see, for instance, "Don't set dere wid yo' head hung down"). Nanny's speech, more heavily inflected speech with southern Black dialect, is a reflection of her more traditional values. In between Nanny's description of her distaste for Johnny Taylor are Janie's thoughts on the matter: "Nanny's words made Janie's kiss across the gatepost seem like a manure pile after rain." The focus in this line on "Nanny's words" shows that Janie disagrees not only with what Nanny is saying, but the way she says it. The dialect, here, helps to accentuate the difference between Janie and Nanny.
One of many important instances of dialect in the book, and Hurston's use of it as a literary device, is the following, as Janie begins her story to Pheoby in Chapter 2:
Janie saw her life like a great tree in leaf with the things suffered, things enjoyed, things done and undone. Dawn and doom was in the branches. 'Ah know exactly what Ah got to tell yuh, but it’s hard to know where to start at.'
There is an interesting contrast here between Janie's speech and that of the narrator. In standard English, the narrator describes Janie's life, that she feels like "a great tree in leaf." This represents a full understanding of Janie's whole life and her perception of it; it is communicated to the reader in standard English. This story of Janie's life is the content of the story which she is about to tell to Pheoby; it is "exactly what Ah got to tell yuh." Janie tells Pheoby that she doesn't know how to tell her story, but the standard-English narrator summarizes it quickly in one quick metaphor. Exactly who this narrator is remains unclear in the story. This tension in how the story is told, with Janie's story, usually in dialect, interspersed with gaps filled in by a seemingly omniscient standard-English narrator, carries through the book. It is perhaps this tension that Hurston alludes to when Janie says that "it's hard to know where to start at." Janie begins her speech not sure of her story, even if the narrator is, in standard English.