What does Tom discover about Casy and how is Casy different from what he once was in Grapes of Wrath?
You can look up the full question here; it's question 11 from Chapters 19-30.
Tom discovers that Jim Casy is a union organizer. In contrast to the way he was when Tom last saw him, he is full of purpose and determination. When Casy is attacked by men whose objective is to halt his union activity, Tom reacts with fury and fights back with deadly force. He grabs the pick handle with which Casy has been killed and "club(s) a guy", perhaps killing him.
Jim Casy was a preacher, but by the time he set out west with the Joad family, he had given up his original calling. Although people still looked up to him as a preacher, he could neither preach nor pray. Bewildered by the injustices he saw around him, Jim Casy was not sure exactly how he should respond. Driven by love for his fellow man but uncertain of his mission, he took the blame for a crime he did not commit, and was sent to prison.
It is while he is incarcerated that Jim Casy finds his true calling. He understands now that "it's need that makes all the trouble" in life, and has discovered that when men join together to stand up for injustice, they have power far greater than they would were they to stand up alone. Casy tries to explain what he has learned to Tom, but Tom is looking at the issue - striking and strikebreaking - from the opposite perspective. He points out the natural tendency of men who are trying to survive by using the example of his own father. Pa must look out for himself and for his family, and is not likely to "give up his meat on account a other fellas", especially when he and they are so close to starvation themselves.
When Casy is killed, Christ-like, by men who "don't know what (they're) a-doin'", Tom reacts with rage. He doesn't see how, practically speaking, the union activity Casy espouses will ever work, but he knows that Casy is "still a - good man" (Chapter 27).