Mr. Horace Gilmer is the prosecutor of the Tom Robinson trial. He provides a contrast to Atticus as he is opposing him in court. Mr. Gilmer represents the traditional racist values and attributes of the South at the time the story is set.
Gilmer is an outsider—he comes from Abbotsville. Scout notes that he seemed to gain an advantage over others as he had a ‘slight cast’ in one eye, which meant that one could not be sure of where he was looking, or at whom—
Thus he was hell on juries and witnesses.
Scout feels sorry for Mr. Gilmer as his witness, Mr. Ewell, is flippant in the witness box. She notices that he has a particular approach to questioning:
Just-in-your-own-words was Mr. Gilmer’s trademark.
He is not sharp enough to follow Atticus’s line of questioning about which hand Mr. Ewell uses. He does not seem to have the respect for his witnesses that Attics has, as Mayella and Bob Ewell are both difficult to manage.
Mr. Gilmer is clear in addressing Mr. Ewell, but when Tom is questioned, Mr. Gilmer refers to him as "Robinson" or "boy." It is Mr. Gilmer who elicits the words from Tom which seal his tragic fate—
"You felt sorry for her, you felt sorry for her?" Mr. Gilmer seemed ready to rise to the ceiling.
Mr. Gilmer comes upon the scene in Chapter 17 of To Kill a Mockingbird. Scout refers to him as the "solicitor." In the context of Scout's narrative, this term may suggest that Mr. Gilmer is a specialist in certain areas of the law and is very qualified.
Scout further describes Mr. Gilmer as having an odd eye that seems to be watching the jury at all times. Because the jury has the impression that they are
...under close scrutiny, [they] paid attention; so did the witnesses, thinking likewise.
Nevertheless, despite his expertise, Mr. Gilmer has trouble with Bob Ewell when this man is put on the witness stand. Ewell's disrespectful language and flippant attitude thwart Mr. Gilmer's intent when he instructs Ewell to answer "just in your own words."
Later, when Tom Robinson is on the witness stand and he speaks "just in [his] own words," Mr. Gilmer repeats Tom's words for the purpose of the prosecution, knowing that in the Jim Crow South no juror (they are all white men) would like this black man's choice of words: "Mr. Gilmer paused a long time to let it [Tom's phrase] sink in."
As the prosecutor, Mr. Gilmer continues to question Tom about Mayella's version of events, and the now-cautious Tom Robinson can only safely say that Mayella "was mistaken in her mind."