One of the most fascinating aspects of the lynch mob scene in Chapter 15 of Harper Lee's To Kill a Mockingbird concerns the fact that Atticus is actually terrified but doing an excellent job of putting up a brave front and trying to take command of the situation.
His first attempt to take command of the situation is hanging a light in front of the jailhouse door to shine directly on him. As the light shines, he strikes a very relaxed pose, sitting in a chair in front of the door reading a newspaper, as we see in Scout's following description:
A long extension cord ran between the bars of a second-floor window and down the side of the building. In the light from its bare bulb, Atticus was sitting propped against the front door. He was sitting in one of his office chairs, and he was reading, oblivious of the nightbugs dancing over his head. (Ch. 15)
The light when the mob was expecting darkness would create intimidation as it highlighted Atticus guarding the door. In addition, his relaxed pose would stand in stark contrast with the mob's heightened emotions, which would also be very intimidating.
Yet, despite Atticus's attempt to look calm in order to intimidate the mob and gain control of the situation, Atticus's true feelings of fear emerge the moment his children come into view. When Scout runs to him, she notes a "flash of plain fear" emitting from his eyes. In addition, Atticus rises to his feet the moment his children arrive on the scene. Scout notes that he moved very slowly, put down the newspaper "very carefully," and straightened the creases of the newspaper "with lingering fingers"; yet, she further notes his fingers "were trembling a little."
One of the other fascinating aspects of the mob scene is that it is Scout who saves the day with her naivete. Scout naively reminds the mob's leader, Walter Cunningham Sr., of his humanity by asking after his son, her schoolmate, and how he was getting along with his entailment, an entailment Atticus has been legally assisting him with. Once Cunningham is reminded of his humanity, he kneels down before Scout, puts both hands on her shoulders, and says, "I'll tell [my son] you said hey, little lady"; then, he commands the rest of the mob to clear out and go home.