The setting and action of chapters 14-15 is pre-trial. During this time you see a couple of different mobs surround Atticus trying to make him know how wrong he is. You also see Atticus of all people sneaking out in the darkness. That's not like him. Atticus never uses a car either, and he strayed from this rule which makes us wonder.
When the town goes in front of the Finches house, Scout and Jem watch the involvement of the different classes in the trial.
During the trial, the mood and suspense build as testimony conflicts with the evidence, and with other testimony. For example, Mayella's story and Tom's story ultimately conflict.