Scout has already revealed some information about the Ewells before the start of the trial. We know that Atticus considers the family "the disgrace of Maycomb for three generations," and that they live in a decrepit house by the dump. Young Burris is a filthy child who shows up on the first day of school ridden with lice and then curses the teacher before walking out of the classroom. But in the courtroom, she gains a new perspective about Bob and Mayella. When Bob tells the packed courtroom that Tom was "that nigger yonder ruttin' on my Mayella!", she sees that
With one phrase he had turned happy picnickers into a sulky, tense, murmering crowd...
Bob "reminded me of a deaf-mute." His "smugness" soon disappeared,
... replaced by a dogged earnestness that fooled Judge Taylor not at all.
Her most important discovery is that Bob is left-handed, making him the most obvious candidate to have beaten Mayella. When Mayella takes the stand, Scout asks Jem "Has she got good sense?" Scout sees that Mayella is "terrified" of Atticus, and that many of her comments seem to contradict each other. After she refuses to answer any more questions, Scout decides that
... if she hadn't been so poor and ignorant, Judge Taylor would have put her under the jail for the contempt she had shown everybody in the courtroom.
Later, it is revealed that Bob spits in Atticus' face and threatens him; that Bob gets a job and loses it within the week; that Bob has been stalking Tom's widow, Helen; and that Bob had probably been the prowler Judge Taylor discovered outside his back porch.