In Harper Lee's To Kill a Mockingbird, confrontation, fear, and deception are key themes that are woven throughout the narrative. When Atticus is assigned the case to defend Tom Robinson—a black man who has been accused of rape by a white woman, Mayella Ewell—he knows that the odds are not on his side. He also knows that Bob Ewell, Mayella's father, is a heavy drinker and is physically abusive to his children.
Atticus never outwardly expresses fear of Mayella testifying; however, he is very astute and observant and knows that there are two things he must balance when cross-examining her during the trial. First, he is aware that the jury is comprised entirely of white men. These men, even if they do believe that Tom is innocent, have the weight of their whiteness pressing on them as a responsibility. If they find Tom innocent, each man will be judged by his peers in Maycomb and may even be physically or financially impinged as a result. Second, because he is certain that Bob Ewell regularly beats his daughter and is responsible for her black eye visible to the jury, he knows that his pointed and direct cross-examination of Mayella could result in her being further punished and injured by her father. Atticus also knows that defending an older man against a young woman who has claimed she was raped is in itself a difficult task; this is even more difficult to defend when the reader considers that Mayella's father fully corroborates her testimony.
In the end, Atticus maintains his integrity and is both courteous to Mayella and true to his defense of Tom. Later in the novel, we do see ramifications as a result of Atticus defending a black man and for revealing the true nature of Bob Ewell. Atticus's comments in those scenes, however, make it clear that he was aware of the risk that he would be undertaking when he agreed to take Tom's case. He could not, though, guess the extreme outcome of Bob Ewell attacking Jem and Scout and Bob's being killed in self-defense as a result.
Atticus could be afraid that the jury will believe that her lies are true just because she is white. It is obvious to the reader that she is lying but the narrator has to use language like her performance or recital to let us know that she has practiced what she has to say. If you are telling the truth, you don't have to practice it, you just have to remember it. The jury doesn't have either the benefit of living 80 years after the incident like we do, nor do they have the benefit of having the narration of Harper Lee. All they have are Atticus' leading questions and Mayella's white testimony. Atticus fears the jury assuming lies to be truth.