Mary Warren has declining values. She gets caught up in the play acting. She begins to enjoy her role in the court hearings. She feels powerful in that the court is depending upon her actions to find people guilty:
Mary Warren is the Proctors' servant who seems timid and subservient but who finds a powerful role in a kind of people's jury in the courtroom.
When John Proctor puts pressure on Mary to tell the truth, she almost gives in to the pressure:
Proctor convinces her that she must expose Abigail's lies to the court, which she agrees to do.
She begins to tell the truth. Then Abigail's presence causes Mary to return to pretending. Mary is easily swayed to follow Abigail's lead. She does not stick to her values and morals. She gets caught up in the play acting. She lies in court. She accuses innocent people of dealing in witchcraft. She knows she is lying but she does not change her story. She is responsible for John Proctor's arrest:
She becomes hysterical before the court, however, and soon joins Abigail in pretending that mere is evil witchcraft at work. Her behavior in the court contributes, in part, to John Proctor's arrest.