In a word, cocky. Mary goes from cowering servant to having a new sense of maturity and purpose when she appointed to be an "official" of the court.
Mary tells John that the court has decided to "spare" Goody Good because she is pregnant. She believes, as she tells John, that "it's God's work we do." She naively believes he will see her in a new light. "You must see it, sir."
She places her new found responsibility above any obligation she has to John, saying, "So I'll be gone every day for some time. I'm -- I'm an official of the court."
John, who loathes the court, does not take kindly to her announcement or defection. He comes after Mary with a whip. But she stands her ground. Miller writes that though "terrified," she stands "erect, striving for her authority."
Mary goes on to tell John that it was she who defends Elizabeth against the accusations of witchcraft which have been leveled against her. Unmoved, John orders Mary to "go to bed." She replies, (with a stamp of her foot that undermines her supposed maturity), "I'll not be ordered to bed no more. I am eighteen, and a woman, however single!"