The protagonist, Grant Wiggins, does not especially want to help Miss Emma. Despite being a college educated man, he is still considered inferior by whites in his society. Powerless to change this disturbing social status quo, Grant becomes an embittered and cynical man. He does not see any hope in fulfilling Miss Emma's request to 'make a man' out of her incarcerated son. Miss Emma's son, Jefferson, is in prison awaiting execution for a crime he did not commit.
In Chapter 6, Grant meets with Henri Pichot and the sheriff, Sam Guidry. The purpose of this meeting is to petition Guidry to allow Grant to visit with Jefferson. Grant goes up to the Pichot plantation for the meeting, but the white men keep him waiting for two and a half hours.
To answer your question about rhetorical devices, I will list three examples below:
1) Grant's use of the word 'doesn't'
Colloquial language is the kind of language we use in informal conversation. During the Jim Crow era, blacks were considered intellectually inferior and socially inferior to whites. In this chapter, Grant talks about 'being too smart.' What he means is that both blacks and whites were expected to speak in language befitting their station in life. Doing otherwise, especially using proper diction and grammar, was considered as evidence that blacks were reaching beyond their station in life.
Grant tells us that he uses the word intentionally the second time because he really does not want to meet with Jefferson. He is hoping that his use of the correct grammatical contraction for 'does not' will so irritate Guidry that Guidry will deny him the private meeting with Jefferson. Ironically, his plan falls through when Guidry continues with his questioning. What Grant has actually done is to succeed in motivating Guidry to subconsciously treat him as an equal, just by his use of proper diction.
This is just a big word to mean juxtaposing (another big word meaning 'to put side by side for comparison purposes') two opposing ideas in a clause or phrase. Example from Chapter 6 :
"She asked me to go to him, Sir. Her idea. Not mine."
Grant and Guidry battle in a linguistic tug-of-war. Notice that Grant is pretty consistent in his answers regarding his decision to meet with Jefferson. He always states that it is Miss Emma's idea. Grant's consistency is actually flattering to Guidry. Recall that Jefferson's own defense attorney characterized his client as a hog in a well-meaning but disastrous attempt to earn Jefferson his freedom. With Grant characterizing the idea of making a man out of Jefferson as a hopeless and impractical mission, the goal of acquiring consent actually succeeds. How? By agreeing with Guidry, Grant is actually reinforcing Guidry's own self-importance; this is actually very flattering to Guidry.
In literature, authors often allow a character to leave a sentence unfinished; this has the beautiful effect of drawing the reader into the action. We become involved when we can interpret the silence of characters based on experiences that are unique to us or our perception of that particular character. Now, Grant remains silent to a question twice in Chapter 6. Although this is not strictly aposiopesis, silence can be used as a literary/rhetorical device to make a point.
Grant remains silent when 1) Guidry accuses him of being 'too smart.' 2) When Guidry reiterates that he is not inclined to like Grant's request.
Grant's silence leaves Guidry's statements unchallenged. Again, this is a great tool to disarm the sheriff.