Then there is a double motive of both Mayella and her father. Tom spurred Mayella’s advances – which made her angry and want to punish him. Also, the book clearly points out an incestuous relationship between Mayella and Mr. Ewell – and Ewell caught Mayella trying to seduce Tom – he was angry with them both and jealous, so he beat up Mayella and then accused Tom of rape.

Mayella’s behavior during her testimony – jittery, defensive and then bursting into tears – in indicative of someone who is not telling the truth.

That’s all I can think of – I keep getting the book mixed up with the film and the book had far more details. Most of the above is found in the chapter that covers the trial near the end of the book.