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?
Mary Warren: I am bound by law, I cannot tell it. To Proctor: I only hope you’ll not be so
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!