Mary Warren, servant to John and Elizabeth Proctor, is described when she first enters in Act I as a "subservient, naive, lonely girl." Throughout the play her words and actions support the idea that she is a lonely girl who just wants to be seen as important, but isn't willing to get in trouble for it.
In Act I she says:
Abby, we've got to tell. Witchery's a hanging' error, a hangin' like they done in Boston two year ago! We must tell the truth Abby! You'll only be whipped for the dancin' and the other things!
Here we see that Mary Warren wanted to be a part of the group in the woods. While she was present during the activities she did not participate in them. Now that their punishment is looming, she is quick to point out Abby's actions and to beg her to confess.
In Act II we see a new Mary Warren. She has been working in the court and the position has gone to her head. For the first time, she stands up to Mr. Proctor when he sends her to bed. For the first time in her life, she feels powerful. Of course, and soon as Proctor stands up to her, weak Mary returns and she goes off to bed.
I'll not be ordered to bed no more, Mr. Proctor! I am eighteen and a woman, however single!
In Act III, Proctor drags Mary Warren to the court to recant her testimony. His wife has been charged and he is no longer willing to stand by and wait for the court to realize the girls are lying. At first, she reads her deposition as told and tells the court that she lied, but when Abigail and the other girls turn on her she crumbles. This makes her turn on Proctor.
I will not hang with you! I love God. I love God.
He came to be by night and every day to sign, to sign, to sign-