At the beginning of the play, Reverend Hale is a man who is pleased to be helping the people of Salem. He is a pretty confident in his skills as a diagnoser of spiritual woes, and feels pretty self-important and spiffy to be called into town as an "expert." When he seems to uncover a nest of witches, he is even more pleased to be front and center in the verdicts and accusations.
As time wears on, and as more and more accusations come, he is still avid, but tired. In Act Two we see him as a diligent man, trying to seek out truth, and trying to see the nature of people before they get to the courts. He comes to the Proctor household and asks them some of the most logical questions of the entire play. At the end, after hearing of Abby's statement to John, he gives John and the other concerned husbands advice. He tells them that if it isn't witchcraft, then some other foul deeds must be going on--he encourages them to think of any possible reasons people might be accusing others of witchcraft to get them out of the way. The men do this, and come prepared to the courts in Act Three.
In Act Three, we see Hale go through a transformation. He is still there in the courts, but is questioning. He tries to challenge Danforth and the others occasionally. He is the only one that stands up for the logic behind Proctor's claims, the only one to defend Proctor, and the only one to point out the glaringly obvious fact that Elizabeth's lie "was a natural lie to tell" about her husband. In Act Three, he is there, but doubting, and by the end of it, has had his eyes opened to the insanity of the courts. It is then that he declares, "I quit this court" and walks away.
In Act Four, we see him weighed down with the guilt of having signed so many false death warrants. He tries to assauge that guilt by getting people to confess, so that they won't die. He realizes that they were falsely charged, and realizes that they are being hanged for nothing. He wants desperately to stem the tide of death, and goes about vigilantly preaching to help people feel okay with confessing. From avid participant of the courts to avid protester of the courts, Reverend Hale goes through a complete turn-around in the course of the play. I hope that helped; good luck.