What does the reader learn from Bob Ewell's evidence? chapter 17 of To Kill a Mockingbird by Harper Lee

One of the methods of characterization is the use of dialogue.  When Harper Lee has her character, Bob Ewell on the witness stand, the reader, both through the eyes of the narrator Scout as well as from his/her own perceptions, learns several things.

First of all, it is apparent that Bob Ewell has decided upon what he is going to say long before he is called to the witness stand.  However, he is not astute because he has not considered the fact that Tom Robinson could not have blackened Mayella's right eye.  Also, his cunning is inadequate as he has not considered the black eye nor has he rehearsed the statements he will make with Mayella in order to be certain that their proported testimony matches.

Also, that he wishes to prove that he and his family are superior to the Robinsons, Ewell makes such statements as his description of where they live:

I knowed who it was, all right, lived down ounder in that n--nest, passed the house every day.  Jedge, I've asked this county for fifteen years to clean out that nest down yonder, they're dangerous to live around'sides devaluin' my property--

That Bob Ewell has no respect for his daughter is indicated in his testimony by his choice of words.  He says that Tom was "ruttin' on my Mayella!" and that he heard her "screamin' like a stuck hog inside the house--"  Ewell is obviously very crude and very ill-educated.  In addition to his diction that clearly indicates his ignorance, he does not understand why the jury and Mr. Gilmer are interested in what hand he uses to write.  When Atticus asks him if he is ambidexterous, Ewell glares at him and replies,

I most positively am not, I can use one hand good as the other.  One hand good as the other.

He does not realize that he has sealed the truth of his cruel treatment of his daughter as well as having raised doubt in the credibility of his testimony since he has lied about Mayella's right black eye.

  Still, in his twisted pride, he "regarded Atticus with haughty suspicion" since he suspects that Atticus will try to trick him (not realizing that he has already been tricked).

Indeed, with the dialogue in Chapter 17, the reader learns much about Bob Ewell.