What are the characteristics of the mob in To Kill A Mockingbird?

The mob, composed of the Old Sarum Bunch, meets the specifics of what is termed mob mentality.

After the sheriff and business leaders of Maycomb visit Atticus, he decides to take a light and position himself that evening in front of the jailhouse door, behind which Tom Robinson is held. When "four dusty cars" stop in front of the jail, Atticus prepares to meet a mob. At first, no one gets out of the cars; then, the men step out in ones and twos. These men reek of stale whiskey and they speak "in near-whispers." Some wear hats pulled low over their eyes.

This group of men exhibit characteristics of the mob mentality in their unrecognizable individual appearances, their coming in a group for strength and anonymity, and their being "liquored up" in order to be less inhibited in their behavior. In addition, there is an emotional excitement generated that Atticus senses because his hands shake as he folds the newspaper that he has been looking at when the men arrive. 

Scout's addressing Mr. Walter Cunningham breaks the anonymity and group thought required for mob action. When she asks him, "How's your entailment gettin' along?" (Ch. 15) and reminds him that his son Walter is in her class, she causes Mr. Cunningham to become uncomfortable about antagonizing Mr. Finch, who has always treated him so fairly. At first, Mr. Cunningham's face is impassive, but soon after hearing Scout's words, he squats down to her and, holding her by the shoulders, he says, "I'll tell him you said hey, little lady." (Ch. 15)

By individualizing Mr. Cunningham, Scout has broken through the dehumanizing effect and anonymity of mob mentality; she has also diffused the emotion of the moment. Now, Mr. Cunningham is one thinking man looking at another and at his daughter.