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.
HECK TATE. Sheriff Tate is seen as an honest, hard-working lawman. Although we later find that he has no love for Bob Ewell, Heck is on a first-name basis with him. Heck also shows that he is no great thinker: He failed to call a doctor to authenticate Mayella's injuries (and possible rape), and he didn't figure out that the bruises that went all the way around Mayella's neck could not have been caused by the crippled arm of Tom Robinson.
BOB EWELL. Racist and unrepentant, Bob legitimizes his reputation as the "disgrace of Maycomb County." He seems happy to be in the spotlight, taking pleasure in accusing Tom of rape and causing an uproar in the courtroom. It is easy for the reader to recognize his lies, and being exposed as left-handed, Bob becomes the obvious culprit of attacking his own daughter. But Bob feels confident, and correctly so, that his tale will be believed by the all-white jury. He knows that his word as a white man will trump the testimony of Tom.
MAYELLA EWELL. A pitiful, lonely young woman, Mayella seems lost in the world of the courtroom. Her testimony reveals that she has no friends and spends her life tending to the rest of the younger Ewell children. She is confused, and her testimony conflicts not only with her father's but her own. It is evident that she often fails to tell the truth, and the reader can see that her fear of her father outweighs her obligation to honestly deal with the situation into which she put herself.
TOM ROBINSON. Although we never know for sure if Tom is innocent of the charges, his testimony is wholly believable. He seems to be an honest, sincere family man, caught in a no-win situation. He is simple-minded, but his motives appeared to be strictly out of a sympathetic urge to help the pitiful Mayella. Strong and robust, aside from his bad arm, Tom is a strong, physical specimen with an even bigger heart.
Tom Robinson, who at the time of the trial was 25 years old, married with three children, and had once been arrested for disorderly conduct, shows himself to be a generous, kind individual based on his testimony. He testified that when walking by the Radley property, which he had to do everyday to get to work, Mayella would always ask him to do odd jobs for her. He gladly did it because he felt sorry for her. He said he never asked for money in return because he could he could see that Mayella was poor and that none of the Ewell children or Bob Ewell were helping her with anyting. We also learn that because he lost use of his left hand in a coton gin accident, there is no way he could've inflicted the injuries Mayella sustained on the right side of her face during the alleged attack and rape.