In Chapter 15, Atticus goes to guard the jail while Tom is imprisoned there. He knows that a mob might show up for Tom and he is correct. Atticus does have Mr. Underwood watching his back, but it still takes courage to confront the mob when they arrive. Atticus tells Walter Cunningham Sr. to take the mob home:
“You can turn around and go home again, Walter,” Atticus said pleasantly. “Heck Tate’s around somewhere.”
Walter says that Tate is off hunting, but Atticus stands his ground. Ironically, it is the children who show up and persuade the mob to leave.
In his summary at the end of the trial, he boldly points out that all of the state's witnesses (with the exception of Sheriff Tate) have a racial bias. This is central to his case, but it is a brave (and necessary) move in order to complete his point that the case has been motivated by race and if Tom is convicted, it will be a travesty of justice, also motivated by racism:
The witnesses for the state, with the exception of the sheriff of Maycomb County, have presented themselves to you gentlemen, to this court, in the cynical confidence that their testimony would not be doubted, confident that you gentlemen would go along with them on the assumption—the evil assumption—that all Negroes lie, that all Negroes are basically immoral beings, that all Negro men are not to be trusted around our women, an assumption one associates with minds of their caliber.
In other words, Atticus bluntly points out that the state's witnesses have used racist motives to frame their story. He is saying to the jury, "if you go along with this racist presentation, then your are no better than they are." He challenges the jury to consider their own racist tendencies. It's a bold, but necessary move.