The word tension, as a literary term, can be defined as the "dramatic or even melodramatic elements of plot, setting, or character" that move a story towards its climax (University of Richmond Writing Center, Writer's Web: First Drafts, "Tension"). By "melodramatic," we mean excessively dramatic. Such dramatic and melodramatic elements serve to intensify the action, the uncertainty of the outcome of events, and even the emotions of the audience. Author Harper Lee uses many narrative elements to create a web of tension leading up to Tom Robinson's trial, and some of those elements are specific events.
Author Lee first creates a web of tension by weaving into the story the mystery surrounding the Finch's neighbor Arthur (Boo) Radley. Arthur Radley behaves contrary to society by never leaving his house, and his contrary behavior has led to the development of many rumors and myths about him, aimed at trying to explain his behavior. The neighborhood children have become so terrified by these rumors and myths that they believe him to be a dangerous but also curious person. As the children try to assuage their curiosity by trying to find out what he looks like, they put themselves in situations in which they feel they must run for their lives, which leaves the reader feeling tense.
Aside from the mystery surrounding Arthur Radley, tension begins to be built once Atticus accepts the role of Tom Robinson's defense lawyer, and the children begin being subjected to ridicule from the people of the town. One of the tensest moments is when Mrs. Dubose insults the children by saying, "Your father's no better than the niggers and trash he works for!," a comment that infuriates Jem to the point that he destroys Mrs. Dubose's garden of camellia bushes (Ch. 11).
The moment of greatest tension prior to the trial is when Atticus is approached by mobs, first one led by Sheriff Heck Tate in front of the Finches' house and the second led by Walter Cunningham Sr. in front of the jail. Though Atticus tells Jem the first mob was made up of nothing more than a group of their friends, it is very clear that the mob's members disapprove of Atticus's determination to defend Robinson, as they move in on him in anger the moment he asserts, "[Robinson's] not going [to the chair] till the truth's told ... And you know what the truth is" (Ch. 15). The members of the second mob, led by drunken Cunningham Sr., intended to take justice into their own hands by lynching Robinson before he could stand trial and probably would have hurt Atticus to accomplish their goal. However, Scout with her innocent neighborly comments directed to Cunningham reminds him of his humanity, and the tense situation diffuses as Cunningham breaks up the crowd, telling them all to go home.
Since all of these events intensify the action and the reader's emotions, we know Lee is using these events to weave a web of tension that leads up to the trial.