Jem is expecting trouble. He has a bad feeling that something is wrong and his father is in danger.
Scout says that, “Jem's got the look-arounds,” which means that he is worried about something. Jem goes to find Atticus, and he is not in his office. The children realize that he is somewhere else, so they keep looking until they find him outside the jail where Tom Robinson is being kept.
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)
What happens next is important. Scout and Jem have to watch their father face down a mob of angry men who want to lynch Tom. Both children demonstrate bravery that night, and so does Atticus.
Jem watches his father very closely. He looks to his father to know how to behave in different situations. For example, on the night that Miss Maudie's house catches fire, Scout starts to panic and Jem calms her down by saying the following:
"In a group of neighbors, Atticus was standing with his hands in his overcoat pockets. He might have been watching a football game. Miss Maudie was beside him.
'See there, he's not worried yet . . . Let's not pester him, he'll know when it's time,' said Jem" (70).
This passage shows that Jem looks to his father to know when it is time to worry. There are multiple times throughout the book where either Atticus or Jem says, "It's not time to worry."
On the night before the Tom Robinson trial, however, Atticus says "good night" instead of "It's not time to worry." Furthermore, the children see him acting differently by taking an extension cord with a lightbulb on the end of it, putting it in the car that he never drives except on lengthy trips, and driving away at bedtime. All of these odd behaviors assist Jem to say, "I've just got this feeling. . . just this feeling" (149).
This feeling leads Jem to spy on Atticus on the night that the Cunningham clan goes to the jail to lynch Tom Robinson. Fortunately, the kids find their father reading outside of Tom's cell and end up assisting Atticus as he stands up to the Cunninghams. If it weren't for the fact that Jem always watches his father, he would not have recognized when things were out of the ordinary and that Atticus may have needed help.