In Chapter 15, a group of men (including Heck Tate and Mr. Link Deas) go to Atticus' house at night to discuss potential trouble with Tom Robinson being kept in Maycomb Jail. They are worried someone or some people might try to get to Tom Robinson and harm him or kill him. Atticus leaves to guard the jail. Sure enough, a different group of men showed up, most likely to abduct and/or kill Tom Robinson; clearly a case of taking the law into their own hands. On the other side, Atticus stood his ground and Mr. Underwood was also there to protect the Tom, and the due process of law, (with some help from Scout, Jem, and Dill). It isn't until Scout completely intervenes and starts a conversation with Walter Cunningham Sr. that the mob leaves. Scout appealed to his good nature and mentioned Walter Jr., and was successful.
I looked around and up at Mr. Cunningham, whose face was equally impassive. Then he did a peculiar thing. He squatted down and took me by both shoulders.
"I'll tell him you said hey, little lady," he said.
Then he straightened up and waved a big paw. "Let's clear out," he said. "Let's get going, boys."