When Mary Warren first appears in The Crucible, the stage notes indicate that she's "seventeen, a subservient, naïve girl." Mary Warren is modeled after the historical Mary Ann Warren, born in Salem in 1674 or 1675, who was a servant to John and Elizabeth Proctor.
Prior to and during the actual witch trials, the historical Mary Warren made accusations of witchcraft against people in the Salem community, including Giles Corey—also a character in The Crucible—whom she accused of appearing to her as a ghost. When Mary herself was accused of witchcraft, she turned on John and Elizabeth Proctor and accused them of witchcraft.
In The Crucible, Mary Warren's subservience and loyalties change through the course of the play. At first she's subservient to John and Elizabeth Proctor, for whom she's employed as a servant. Her subservience and loyalty shift to Abigail Williams, the ringleader of the group of girls at the center of the accusations of witchcraft toward others in the community. Finally, Mary is loyal only to her own interests and self-preservation when she's accused of witchcraft, and she accuses others of witchcraft to save herself.
In The Crucible, Mary's first "victim," whom she apparently accuses of witchcraft reluctantly and unwittingly, is Sarah Good, a destitute, sixty-year-old woman considered an outsider by the Salem community.
In act 1, scene 2, Mary tells John Proctor about her experience at the trials that morning, which John Proctor had forbidden her to attend.
MARY. (Innocently.) I never knew it before. I never knew anything before. When she [Sarah Good] come into the court I say to myself, I must not accuse this woman, for she sleep in ditches, and so very old and poor...But then...then she sit there, denying and denying, and I feel a misty coldness climbin‘ up my back, and the skin on my skull begin to creep, and I feel a clamp around my neck and I cannot breathe air; and then...(Entranced as though it were a miracle.) I hear a voice, a screamin‘ voice, and it were my voice...and all at once I remembered everything she done to me! (act 1, scene 2)
In act 2, Mary decides to tell the truth to Deputy Governor Danforth, who's conducting the trial, that she and the other girls were simply pretending when they said that they saw spirits and acted as if they were attacked by witches.
DANFORTH. And you, Mary Warren...how came you to cry out people for sending their spirits against you?
MARY. It were pretense, sir.
DANFORTH. (With great unbelief.) Ah? And the other girls? Susanna Wallcott, and...the others? They are also pretending?
MARY: Aye, sir (act 2, scene 2).
When Danforth confronts Abigail about this, she denies it and instead acts as if she's being attacked by Mary's spirit in the form of a freezing wind, and then she acts as if Mary's spirit is a "yellow bird" sitting in the rafters, intending to attack Abigail's face. Soon the other girls in the room take up the pretense, and all of them cry out against Mary.
Mary, hysterical with fear, backs away from John Proctor, who is trying to help her, and accuses him to save herself.
MARY. (Pointing at Proctor.) You are the Devil‘s man!...I‘ll not hang with you! I love God, I love God—....(Hysterically, indicating Proctor.) He come at me by night and every day to sign, to sign, to...
DANFORTH: Sign what?
PARRIS: The Devil‘s book? He come with a book?
MARY: (Hysterically, pointing at Proctor.) My name, he want my name; I‘ll murder you, he says, if my wife hangs! We must go and overthrow the court, he says...!
...(Her sobs beginning.) He wake me every night, his eyes were like coals and his fingers claw my neck, and I sign, I sign....
(Screaming at him.) No, I love God; I go your way no more, (Looking at Abigail.) I love God, I bless God....(Sobbing, she rushes to Abigail.) Abby, Abby, I‘ll never hurt you more! (All watch, as Abigail reaches out and draws sobbing Mary to her, then looks up to Danforth) (act 2, scene 2).
John Proctor is led away to jail and is accused of witchcraft.