With the main source of conflict in Harper Lee's To Kill a Mockingbird being the actions surrounding the trial of Tom Robinson, tension begins even in part I of the novel.
In Chapter 10, Atticus proves that he is no coward by shooting the rabid dog, and word gets out that Scout will not fight anyone after getting into trouble. But, the main source of tension comes from town gossip:
...the school buzzed with talk about his defending Tom Robinson, none of which was complimentary.
In an example of foreshadowing, Scout and Jem receive air rifles, and Atticus tells thme it is all right to shoot blue jays, but they must never to kill a mockingbird because "it's a sin to kill a mockingbird."
In this chapter, there is much tension as men from town come and stand in the Finch yard, an occurrence that was never mere coincidence. Scout narrates,
In Maycomb, grown men stood outside in the front yard for only two reasons: death and politics.
Sheriff Heck Tate tells Atticus that he is going to move Tom Robinson to the county jail the next day.
Mr. Link Deas said, "Nobody around here's up to anything, it's that Old Sarum bunch I'm worried about...can't you get a--what is it, Heck?"
"Change of venue," said Mr. Tate. "Not much point in that, now is it?"
Atticus will not back down from the men, saying that he could not refuse to defend him. He tells Mr. Deas,
"Link, that boy might go to the chair, but he's not going till the truth's told....And you know what the truth is."
Later that evening, the children follow Atticus and find him sitting in one of his office chairs before the jailhouse door. Scout and Jem also find four dusty cars coming from the Meridian highway, all in a line. As the men approach Atticus, tension clearly rises. The men almost whisper to him,
"You know what we want....Get aside from the door, Mr. Finch."
When Atticus tells the men that Heck Tate and his men are around somewhere, one of the men contradicts, saying that they have been tricked into going into the woods. Then, Atticus says, "...that changes things, doesn't it?""
"It do," another deep voice said.
"Do you really think so?"
This is Atticus's challenge to the men. But, Scout intervenes and diffuses the tension as she personalizes her address to Mr. Cunningham; he then becomes ashamed and tells the men to leave.
In this chapter, Jem and Scout await the verdict, and the Reverend Sykes tells Jem,
"I ain't never seen any jury decide in favor of a colored man over a white man."
Having created a false climax in this chapter, Harper Lee has Scout and Jem frightened as they walk to the school pageant; however, the threat is none as it is only Cecil Jacobs who appears. Then, later in the chapter the children hear someone shuffling behind them. Suddenly, Jem lets go of Scout's hand and is jerked backwards; Scout's
toes touched trousers, a belt buckle, buttons, something I could not identify, a collar, and a face. A prickly stubble on the face told me it was not Jem's. I smelled stale whiskey.
After this, Scout sees someone carrying Jem, whose arm dangles unnaturally. Of course, the real climax and greatest tension comes as Boo and Bob Ewell struggle.