How is Mary Warren's role in the Salem witch trials different in The Crucible from what had really happened? In other words, who does she try to protect before she herself is accused and how is that different from the play?
The historical Mary Warren, while working for the Proctor family, began having seizures and claimed to have seen the ghost of Giles Corey. She posts a note asking people to pray for her. She claims that John Proctor awakens her and torments her about the note. Parris reads the note during a church service and the people discuss this afterward, with some concluding that the girls had lied about seeing the devil. The other girls accuse her of witchcraft and then she is formally accused and confesses. She then begins accusing others of witchcraft (including the Proctors).
The Mary Warren of the play, The Crucible, does not have seizures. Betty is the one who is sick at the start of the play. She also never claims to see Giles Corey's ghost. There is no mention of the church note in the play, but she does admit the girls were lying in court. She says, "It were pretense." When she is being questioned in court, she does say John wakes her up to get her to sign something that will clear Elizabeth's name. In the historical record and in the play, John is pretty harsh with Mary. But it is his way of getting her to tell the truth.
The Mary of the play seems to be a weaker, more innocent girl than the historical Mary Warren. For instance, in the play, when Abby turns on her, Mary does lash out at John for putting her in this position. She says, "You're the devil's man!" The historical Mary goes further, accusing both John and Elizabeth of witchcraft. The historical Mary makes many more accusations. But in the play, at the end of Act 1, it is Abby and Betty who start making numerous accusations. Evidently, Miller toned down Mary's character to make at least one of the girls more sympathetic, thereby allowing the reader to see how she (and all the girls) were manipulated by the fear of being condemned by an oppressively religious community.