How did the author use descriptions of settings, events immediately prior to the trial to build and intensify a mood of suspens?
Everything that has occured so far in the novel has led to the moment of the trial. The question is about chapters 15 to 18.
Harper Lee used a variety of scenes and sub-plots to set up the drama of the Tom Robinson trial in To Kill a Mockingbird. In Chapter 14, Jem and Scout feel the stares and hear the gossip from the people of Maycomb concerning Atticus defending a Negro. Lee adds the runaway Dill to the mix, giving Atticus one more problem to ponder before the trial. In Chapter 15, Atticus meets with a group of concerned citizens who warn him about something that Jem and Scout don't quite understand. When Atticus heads to the jail later that night, the children follow, setting the stage for their dramatic rescue of their father from the prospective lynch mob.
The day of the trial is narrated in great detail by Scout. It is obviously not a normal day in Maycomb. Throngs of people arrive for the trial: Religious zealots, out-of-towners, and Negroes make up just some of the people as "the county went by." Women argured in the street, and the "courthouse square was covered with picnickers" before Jem, Scout and Dill decided to make their appearance as well.
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.