Diction is simply word choice. Since this story takes place in the South during the great depression, several characters speak in unique ways.
Scout and Jem
Scout and Jem are children of the South. They speak often in Southern slang.
"Shoot no wonder, then," said Jem, jerking his thumb at me. "Scout yonder's been readin' ever since she was born, and she ain't even started to school yet. You look right puny for goin' on seven." (ch 1)
This is one of the first real examples of dialogue in the story, and here the diction establishes the setting very well. The words in bold are all examples of carefully chosen Southern diction. Some of the words and phrases are much more common in the South. For example, “yonder” and “goin’ on” are distinctly chosen phrases.
Atticus is one of the few educated people in Maycomb, and this is demonstrated by his choice of words. Of all of the characters, Atticus seems to choose his words most carefully. He is a lawyer, after all.
"They can go to school any time they want to, when they show the faintest symptom of wanting an education… There are ways of keeping them in school by force, but it's silly to force people like the Ewells into a new environment-" (ch 3)
Atticus always speaks clearly and distinctly chooses his words. He does not speak down to his children, and they are more intelligent for it.
Scout notes that when Calpurnia is at church with the other black people she talks differently than when she is at home. Calpurnia is an example of a code-switcher, someone who changes diction based on who the audience is in order to fit in better.
"Baby," said Calpurnia, "I just can't help it if Mister Jem's growin' up. He's gonna want to be off to himself a lot now, doin' whatever boys do, so you just come right on in the kitchen when you feel lonesome…." (ch 12)
Calpurnia’s talk is also a prime example of the Southern dialect, and her diction is a mixture of educated and uneducated, especially when she is speaking affectionately as she is here.