There is always much that is revealed about a person who finds him or herself on the witness stand and under the obligation of telling the truth.
First of all, Sheriff Tate is dressed professionally in a business suit for the trial of Tom Robinson. He answers the prosecutor's questions succinctly, offering no more information than is necessary. When Atticus cross-examines, Sheriff Tate displays his admiration for the acuity of Atticus's mind; for instance, when Atticus asks for clarification of the time of the incident at the Ewell house as well as when he wants to know exactly which eye of Mayella has been blackened, Tate smiles as he recognizes the precision of Atticus.
A hostile witness to begin with, Bob Ewell reveals his ignorance as he attempts to villify Tom Robinson and implicate him as the perpetrator of a crime. His seething hatred is revealed in the pejorative terms that he uses regarding Robinson, as well as his disrespect for his own daughter as he speaks of her as "screamin' like a stuck hog inside the house---" Ewell's attempts to embellish his testimony, in contrast to the minimalist speech of Sherriff Tate, suggest his ulterior motives. His ignorance is evident when he fails to realize why Atticus has him write his name and points to his being left-handed.
In addition, other than having bathed and washed his hair, Ewell displays little respect in the courtroom ashe swears and is crude in his language. He exhibits "a haughty suspicion" of Atticus, suggesting that he has something to hide.
The repressive and abusive conditions under which Mayella lives are clearly evident in this witness's appearance and testimony. When first questioned, she looks at the judge and bursts into tears, indicating the stress that she feels. However, she is not ignorant as her father is because she protests against Atticus,
"Don' wnt him doin' me like he done Papa, tryin' to mke him out left-handed...."
Unlike Sheriff Tate, she embellishes her testimony as does her father. However, Scout points out that Mayella has
something stealthy about hers [testimony] like a steady-eyed cat with a twitchy tail.
Throughout her testimony, Mayella looks to her father and is "jumpy." At one point, Mayella contradicts herself when Atticus asks if her father has ever hit her,
"No, I don't recollect if he hit me. I mean yes I do, he hit me."
This slip of the truth may be an unconscious effort on Mayella's part to seem honest, or, perhaps, a subconscious play for sympathy. At any rate, Mayella Ewell reveals herself as a rather pitiable victim of her environment.
Appropriately aligned to the symbol of the mockingbird, Tom Robinson reveals himself as entirely guileless and decent. He never implies that Mayella has lied; he simply states that she may be mistaken in her recollection. Without realizing the implications of anything that he says, Tom speaks from his heart, the most truthful place in any human being. Unknowingly, he utters words that condemn him regardless of the accuracy and extent of the evidence to the contrary. For, when he says that he has felt sorry for Mayella, he a black man and she a white woman, he has violated the social taboo that supersedes all litigation at the time.