When Atticus stands guard outside Tom Robinson’s cell, Jem refuses to leave even though there is a lynch mob there.
Atticus knows that the group of men will try to come for Tom Robinson, to kill him before he can be tried. Since he is a black man whom they think attacked a white woman, they believe they have that right.
When the children go to check on Atticus, they realize he is sitting outside the jail reading.
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)
At first, Atticus tells Scout to leave Atticus alone and wants to go home. Then they see the mob.
Since there is no light, Atticus had to run an extension cord. When the men ask Atticus if Tom Robinson is inside, Atticus tells them to be quiet because he is asleep—so the men whisper.
When the men tell Atticus that Heck Tate, the sheriff, is out in the woods, he does not seem concerned. Scout is excited, not wanting to miss the fun. She runs to Atticus.
Jem shrieked and tried to catch me, but I had a lead on him and Dill. I pushed my way through dark smelly bodies and burst into the circle of light. (ch 15)
Jem is concerned. He runs after his sister to protect her. Atticus shows “a flash of plain fear” when he sees Scout, and then again when Jem and Dill come out. With trembling hands, Atticus tells Jem to go home. He refuses.
Jem shook his head. As Atticus's fists went to his hips, so did Jem's, and as they faced each other I could see little resemblance between them: … but they were somehow alike. Mutual defiance made them alike. (ch 15)
Jem always does what he is told. Here, he doesn’t. He is worried about his father, and his sister. He stands his ground. Atticus makes “threats, requests” and pleas, but Jem refuses. Scout decides that “Jem had his own reasons for doing as he did” (ch 15).
Scout intervenes, striking up a conversation with Walter Cunningham’s father, while Atticus’s “attention amounted to fascination” (ch 15). They leave, and “Atticus reached out and massaged Jem's hair, his one gesture of affection.” Clearly, Atticus is just relieved that his children are okay.
Jem shows a great deal of maturity in this episode. He risks physical danger and Atticus’s disappointment to protect his sister, and then his father. He demonstrates both physical and moral courage, and the incident ultimately brings Jem and his father closer together.