One thing that you could look at is John's attitude toward Elizabeth in the beginning of the play. He feels that she is constantly hounding him about his affair. This is shown when he has his conversation with her at the dinner table. At the end of the play, he tells Elizabeth that he will not save himself while he watches his friends go off to die. During that scene, he realizes that Elizabeth is supportive and has also paid the price for his sin. The manner in which he speaks to her is much different. Look for the different ways in which he addresses her.
John Proctor decides to stay out of the whole witchcraft hysteria problem, he wants nothing to do with it, and believes that it will blow over.
John Proctor's attitude toward the witchcraft hysteria in Salem changes from the beginning of the play when he refuses to believe that it is a serious problem. He is actually very offended by Reverend Parris's behavior when he summons Reverend Hale from Beverely to come to Salem and diagnose Betty's illness. Reverend Parris believes that his daughter has been bewitched.
"Parris: a wide oopinion's running in the parish that the Devil may be among us, and I would satisfy them that they are wrong." (Miller)
"Then let you come out and call them wrong. Did you consult the wardens before you called this minister to look for devils?" (Miller)
In Act III, Proctor realizes that the devil is working in Salem, but not in person, he is acting through the witchcraft hysteria itself.
"A fire, a fire is burning! I hear the boot of Lucifer, I see his filthy face! And it is my face and yours. Danforth! For them that quail to bring men out of ignorance, as I have quailed, and as you quail now when you know in your black hearts that this be fraud--God damns our kind especially, and we will burn, we will burn together." (Miller)
By Act III, Proctor knows that the court proceedings are the very nature of evil, putting innocent people to death, yet he is helpless to stop it. He gets sucked into the process because of his relationship with Abigail.
Look for your quote at the very end of the play in the scene between John and Elizabeth when she visits him in the jail moments before he is to die. If John will confess, his life will be spared. Although John wants desperately to live and be with his family, he cannot bring himself to save his own life by giving legitimacy to the court, thereby betraying his friends, like Rebecca Nurse, who surely will die that morning. Several lines in this scene point to this fundamental change in John. Considering the sin he committed with Abigail, John is amazed that he has turned out to be such a good man.