The scene at the jail is prefaced with a scene of many men appearing in the Finch yard in Chapter 15 of "To Kill a Mockingbird."  When Atticus comes out to speak to them, they suggest that Atticus have Tom Robinson moved somewhere else because they are worried about the "Old Sarum bunch" who get "liquored up." As the men argue with Atticus, Jem calls to him that the phone is ringing. 

The men jumped a little and scattered....Laughter broke them up.

In Chapter 16, men come again, but it is night and they come to the jail:  "You know what we want...Get aside from the door, Mr. Finch."  These are not the same men; "There was a smell of stale whisky and pigpen about....They were not the people that [Scout] saw last night." 

It is only when Scout says hello to Mr. Cunningham and speaks of "entailments" that the man changes his aspect and tells the others to clear out.  When Scout talks to Mr. Cunningham, she does what Jem did.  She made the men see Atticus as an individual.  This action reinforces the lesson that Scout has learned in the earlier part of the novel:  When one stands in some else's shoes, the one can perceive the other's point of view.  Scout forces Mr. Cunningham to stand in the shoes of Atticus, thus saving her father's life.

Mob mentality depends upon a group frenzy.  But, the people in Macomb are from a small enough town that they are quickly able to see others from an individual point of view.  However, their cultural conditioning does make them adhere to certain prejudices that often supercede their personal relationships with people.