One of the passages from Chapter 18 of To Kill a Mockingbird that depicts the character of Mayella Ewell is the following:
Apparently Mayella’s recital had given her confidence, but it was not her father’s brash kind: there was something stealthy about hers, like a steady-eyed cat with a twitchy tail.
This quote is fairly indicative of Mayella's character. She is more feline in her entrapment of Tom Robinson than the brash Bob Ewell, who has boldly called Sheriff Tate and had Robinson arrested for rape, even though no doctor has been summoned. Previous to the day in question, Mayella has made several overtures to Tom, asking him to help her with various tasks. But, she tries to intimate that Tom's chopping up of the chiffarobe was the first time he was inside her yard. For, when Atticus asks her,
"Was this the first time you asked him to come inside the fence?"
Mayella "jumps slightly" at this question. Then, as Atticus questions her further, Mayella looks around at the court reporter and up at Judge Taylor as though measuring those around her. Further, as Atticus asks her if Tom beat her in the face, Mayella replies,
"No, I don't recollect if he hit me. I mean yes I do, he hit me....Huh? Yes, he hit--I just don't remember, I just don't remember...it all happened so quick."
With this "twitchy" response, one can easily imagine a cat who twitches its tale as it squares off against its prey.