Proctor evolves throughout the play from being a spiritually ambivalent figure to its moral compass. At the start of the play, Proctor is uncertain of many things. He regrets his affair with Abigail, but still lacks the full force of being able to take a stand. As the play progresses, he is able to not only be able to call out Abigail's actions, but he delivers the most impassioned pleas for individuals not capitulate to the political pressure of "naming names," but rise and defend reputations in the name of truth and justice. When he refuses to sign a confession that is false, it becomes a moment when the audience can identify with Proctor, an ordinary man who has been placed in bizarre circumstances and has ended up becoming an extraordinary moral and spiritual figure.
To me, the major change in John Proctor's character is that he goes from being a very troubled man who is wracked with guilt to being one who is serene at the end of the play.
At the start of the play, Proctor feels so guilty because of the fact that he had this affair with Abigail Williams. It haunts him and makes him think very poorly of himself.
By the end of the play, though, he is able to regain a positive self-image. He gets to the point where he feels good enough that he is willing to die to protect his good name.