How does the mood of the courtroom change abruptly when Bob Ewell takes the stand after Heck Tates testimony?

I think there is a sense of heightened excitement and concealed derision as Bob Ewell takes the stand. The audience knows they is going to be a show now that Bob Ewell is on the stand.

“Are you the father of Mayella Ewell?” was the next question. “Well, if I ain’t I can’t do nothing about it now, her ma’s dead,” was the answer. Judge Taylor stirred. He turned slowly in his swivel chair and looked benignly at the witness. “Are you the father of Mayella Ewell?” he asked, in a way that made the laughter below us stop suddenly. “Yes sir,” Mr. Ewell said meekly. ch 17