Looking back at lines 323-344 in Act Two, it appears obvious that Mary's defense of Elizabeth Proctor in court represents a change in the power dynamics among Mary, Elizabeth, and John Proctor:
Mary Warren: They had Doctor Griggs examine her, and she’s full to the brim.
And smokin’ a pipe all these years, and no husband either! But she’s safe, thank
God, for they’ll not hurt the innocent child, But be that not a marvel? You must
see it, sir, it’s God’s work we do. So I’ll be gone every day for some time. I’m -
I am an official of the court, they say, and I - She has been edging toward
Proctor: I’ll official you! He strides to the mantel, takes down the whip hanging
Mary Warren, terrified, but coming erect, striving for her authority: I’ll not
stand whipping any more!
Elizabeth, hurriedly, as Proctor approaches: Mary, promise now you’ll stay at
striving for her way: The Devil’s loose in Salem, Mr. Proctor; we must discover
where he’s hiding!
Proctor: I’ll whip the Devil out of you! With whip raised he reaches out for her,
and she streaks away and yells.
Mary Warren, pointing at Elizabeth: I saved her life today!
Silence. His whip comes down.
Elizabeth, softly: I am accused?
Mary Warren, quaking: Somewhat mentioned. But I said 1 never see no sign
you ever sent your spirit out to hurt no one, and seeing I do live so closely with
you, they dismissed it.
Elizabeth: Who accused me?
sarcastical no more. Four judges and the King’s deputy sat to dinner with us but an hour ago. I -
I would have you speak civilly to me, from this out.
Proctor, in horror, muttering in disgust at her: Go to bed.
Mary Warren, with a stamp of her foot: I’ll not be ordered to bed no more, Mr. Proctor! I am
eighteen and a woman, how-ever single!
Proctor: Do you wish to sit up? Then sit up.
Mary Warren: I wish to go to bed!
Proctor, in anger: Good night, then! Mary Warren: Good night. Dissatisfied, uncertain of herself, she goes out. Wide-eyed, both, Proctor and Elizabeth stand staring. Elizabeth, quietly: Oh, the noose, the noose is up!
Proctor: If she is innocent! Why do you never wonder if Parris be innocent, or Abigail? Is the accuser always holy now? Were they born this morning as clean as God’s fingers? I’ll tell you what’s walking Salem - vengeance is walking Salem. We are what we always were in Salem, but now the little crazy children are jangling the keys of the kingdom, and common vengeance writes the law! This warrant’s vengeance! I’ll not give my wife to vengeance!