In "The Crucible" why does Mary Warren turn against John Proctor? What techniques are used in that scene?

The
hysteria that permeates the town of Salem and the surrounding environs in 's play
, a story intended as anfor the "witch hunts" that took place in
the United States during the late 1940s and early 1950s when fears of communism and the Soviet
Union gripped much of the nation, causes the region's inhabitants to act irrationally and
viciously towards each other. Friends turn on friends; family members turn on family members.
Fear of being accused of sorcery is sufficient to cause people to make ill-founded accusations
in order to deflect suspicions away from themselves. Such is thein which Miller's play takes
place. From its opening scene, in which Reverend Parris frantically tries to deal with his
daughter's apparent coma following her and her friends' peculiar and highly suspicious
activities in the woods, The Crucible depicts a society on the threshold of
a Hell of its own making.

As Miller's play progresses, the hysteria reaches a
fever pitch, and old grievances become the fodder for new ones, all intended to ensure one's
survival. At the center of this drama are Abigail, whose actions precipitate the tragic chain of
events, and, increasingly, Mary Warren, Abigail's replacement in the Proctor home following
Abigail's dismissal for engaging in sexual relations with John Proctor. Proctor's relationship
to Abigail continues to plague him, and he becomes dependent upon Mary's testimony in court to
absolve him while indicting Mary's friend, Abigail. Mary knows of Proctor and Abigail's affair,
and is caught between her employer and her friend. As Miller depicts the scene late in Act
Two:

Mary Warren: . . .Abby'll charge lechery on you, Mr.
Proctor!

Proctor: She's [Abigail] told you!

Mary Warren: I
have known it, sir. She'll ruin you with it, I know she will.

Proctor is desperate to ensure Mary's testimony will exonerate him in the witch trial
that is tearing the town apart. In Act Three, Proctor testifies before the court that Mary is
prepared to confess that charges of sorcery, or witchcraft, have been fabricated, and that her
previous statements were false. As Mary is questioned by Danforth, however, Abigail and the
other girls enter the court room. Danforth questions Abigail regarding Mary's deposition and
testimony, and Abigail accuses her now-former friend of lying when Mary denies any kind of
Satanic activities. Abigail succeeds in turning the tables on Mary and, by extension, John
Proctor. Abigail pretends to be experiencing symptoms of demonic influence, which casts new
suspicions upon Mary, who is now, again, suspected of sorcery. Abigail, as we know from the
play's previous activities, is far more duplicitous in nature than the more simple-minded Mary
Warren, and the latter breaks down, lending credence to Abigail's intrigues.

In short, Mary is sufficiently frightened by Abigail's tactics that her fear
overshadows any commitments she has made with Proctor. 

As noted, Miller
intended his play as an allegory about the anti-communist paranoia that permeated the nation
during the early 1950s. The point he makes in The Crucible, especially
during the trials, is that innocent people suffered unjustly because of the paranoia about
threats that did not, or only marginally, existed. Honest people felt compelled to accuse others
in order to deflect attention away from themselves.