A dynamic character undergoes some change during the course of a novel, play, etc.
In The Crucible, Hale is dynamic because when he arrives in Salem, he's determined to investigate, solve, and prove the presence of witchcraft. However, by the play's end, Hale does recognize the truth which is that Abby and the other girls were lying all the time and that John Proctor is guilty only of adultery.
In the narrative that introduces Hale in Act 1, Miller notes that prior to his arrival in Salem, Hale had one experience with a witch. However, that woman turned out to be a "mere pest." Despite this experience, Hale remained determined that witchcraft did exist.
However, that experience never raised a doubt in his mind as to the reality of the underworld or the existence of Lucifer's many-faced lieutenants. And his belief is not to his discredit. Better minds than Hale's were--and still are--convinced that there is a society of spirits beyond our ken.
By the time Danforth has made up his mind not to retract any of the accusations, it is too late. Also, by this time, Hale has acknowledged that the girls were lying and he realizes that he has also played a part in contributing to the hysteria and false accusations.
Why, it is all simple. I come to do the Devil's work. I come to counsel Christians they should belie themselves. His sarcasm collapses. There is blood on my head! Can you not see the blood on my head!! (Act 4, Scene 2)