Mary Warren as well as all the girls went from being treated as insignificant children to the most significant people in Salem Village by crying out witchcraft upon people.
The exchange between John Proctor and Mary Warren in Act II, Scene 2 sets the stage for Mary's new gained authority. Mary's defiance of John Proctor's directive that she not go to town and be a part of the court is the first example of Mary exerting her power. Children of Salem Village were "meant to be seen, not heard," and by Mary standing up to John Proctor, Miller establishes the lengths that the court will go to prove that there is indeed witchcraft in Salem Village. Mary further demonstrates her authority when she says concerning Elizabeth, "I saved her life today." The arrogance in this statement is the pinnacle of Mary's authoritative power over the adults in Salem Village. She also states that she will be attending court every day, and she is shocked that the Proctor's don't see her importance to the court. This scene clearly shows the transformation of Mary's character from a frightened Salem child to a vindictive authority figure.
Given what she states to John and Elizabeth Proctor, she feels that she has authority given to her by the courts, and probably from God. She tells them "I am an official of the court they say", referring most likely to the judges in charge of running things. So, based on their words, she has taken her "authority" to heart, along with what she feels is God's mission. Of this she states, "I am amazed you do not see what weighty work we do...it is God's work we do...the Devil's loose in Salem, Mr. Proctor; we must discover where he's hiding!" She is most likely parroting phrases from the judges themselves, and gains new-found courage in light of her "importance" in court. She stands up to John, telling him that she'll "not stand whipping any more" and that she'll "not be ordered to bed no more." The only other glimpse that we get of Mary Warren before this is in the first Act where she is whining and shaking in her boots about the potential of being called a witch because of their dancing. Her court-appointed "authority" and self-important mission for God has given her a bit more snuff than she usually displays.