Hale serves as an outside voice of reason to the reader. He is not part of the community of Salem, and he is obviously a very intelligent man. In addition to that, he is not a man who is quick to judge. He wants to see justice done, but he wants it to be done fairly. For that reason, he is a very trustworthy character to the reader.
John and Elizabeth give Hale reason to doubt almost as soon as he enters their house. John mentions that he believes that Reverend Paris is a greedy man consumed with his reputation. John provides evidence of that with the golden candlesticks that Paris had to have in the church.
Shortly after, John also says that he knows it isn't witchcraft.
"But I know the children’s sickness had naught to do with witchcraft."
Hale asks John how he knows this, and John explains that Abigail Williams told him that they were "sportin' in the woods." Hale is shocked to learn this, because those accused have confessed to witchcraft. John points out that makes sense, since the alternative is death by hanging. Hale had suspected the accused were confessing in order to save their lives, and John gives voice to that suspicion.
Hale is also shocked by the number of people being accused along with who is being accused. Hale doubts Elizabeth's guilt, but when Francis Nurse enters and tells everybody that Rebecca has been charged, Hale's certainties really begin to falter.
When Cheever shows up to arrest Elizabeth, and Hale sees the emotional rip current that John and Elizabeth go through, Hale just about completely changes his initial position on the court proceedings. Elizabeth is focused on her children, and John vehemently defends his wife. Hale can't help but be caught up in their emotional pleas to him and to each other.
Once Hale gets to intimately know Elizabeth and John, a change emerges in his characterization. As he changes his perspective on what is happening in Salem, it becomes easier for us to share his doubts.
Hale arrives in Salem convinced that witchcraft is being practiced. In Act II, Hale sits down with the Proctors and talks to them about their own lives in Salem. Before leaving, Hale is "struck" with the revelation that the children's sicknesses have "naught to do with witchcraft." Hale is slowly beginning to have doubts, encouraged by Proctor's revelation. He recognizes that the machinery of the trials might be operating outside of divine will:
Proctor: And why not, if they must hang for denyin’ it? There are them that will swear to anything before they’ll hang; have you never thought of that?
Hale: I have. I - I have indeed. It is his own suspicion, but he resists it. He glances at Elizabeth, then at John.
Talking with John and Elizabeth has begun the process of slowly changing Hale's perspective on what is happening in Salem.
In addition to this, Hale begins to see the emotional intensity between John and Elizabeth. Francis Nurse and Giles Corey enter the scene. They are shocked and saddened their wives have been apprehended. This experience is repeated between Proctor and Elizabeth, when she is arrested. Hale is taken aback by the emotion of Elizabeth being arrested. Proctor's intensity to prove his wife innocent leaves an impression upon him. These emotional revelations contribute to changing his point of view.
As Hale changes because of what he sees, so do we. When we see how Elizabeth bids farewell to her children and how Proctor vehemently defends her, we recognize the human cost to the trials. We begin to share Hale's "suspicion" that the system is out of control. As so many people are being apprehended, Hale's suspicions are our own.