Mary Warren begins the play as a rather meek and timid girl. When she returns from Salem, after having been all days at the trials, she is proud and defiant. John Proctor specifically forbade her going to Salem, and Mary defied him, and told him she had to, that she was working for the courts now and doing the work of God. She also tells John Proctor she will be going back to the court and will not be doing her work around the house until the trials are over. For a woman to stand up to a man, especially her employer, like this was improper and grounds for a beating in Purtain time.
Mary saves herself a beating by telling John that Elizabeth's name has been brought up at the trials. Mary chooses a great time to reveal this information--right as John is about to beat her. Mary also gives Elizabeth the poppet, which is a strange gift for an other woman. Mary has also become sneaky and furtive, plotting and planning along with Abigail.
Mary eventually goes to bed, announcing that doing God's work was tiresome. She goes from being the lowest member of the Proctor household to announcing herself as the role of God's messenger and the voice from Salem--defying John and ignoring her household duties.
When John learns that Mary Warren has gone to Salem against his wishes, he asks Elizabeth why she let her go; Elizabeth tells him that she couldn't stop her, and that Mary is "a mouse no more."
Mary arrives at the Proctor home after her day in Salem, and she claims her "insides are all shuddery" from being in the court all day. It is clear that Mary is conflicted; she gives Elizabeth a gift of a poppet and promises to get up early to clean the house. She weeps when she tells the Proctors about the 39 people sentenced to hang. However, moments later, Mary becomes indignant when she describes Sarah Good's mistreatment of her, then weakens when John and Elizabeth challenge her, only to pivot again to a defiant stance when John threatens to whip her.
Mary tries to wield her newfound authority as a court official, but she is not very convincing. By the end of the act, the true source of her conflict is clear: she doesn't want either of the Proctors implicated in wrongdoing, but she mortally fears backlash from Abby and her followers. After Elizabeth is arrested, John roughly demands that Mary tell the court that Abby and the other girls are lying. Mary confesses, "I cannot, they'll turn on me."