This has already been shown in stage directions in Act 2 when she claims to Proctor that what she does in court is Gods will and that she will not take being treated as a child by Proctor any longer, all the while acting uncertain. backing away from him, but keeping her erect posture, striving, striving for her way. She does not always know what she wants and tries to stand up for what she thinks she wants. A good example of this is in Act 2 when Proctor orders her to bed and she declares Ill not be ordered to bed no more, Mr Proctor!
I am eighteen and a woman, however single! to which Proctor replies Do you wish to sit up? Then sit up. And Mary ends up contradicting herself I wish to go to bed! The audience feels that she wants to stand up for herself again in Act 3, because she wants to tell the officials the truth but then ends up choosing the easy path and turning around to blame Proctor when she is pressured to much to stand up for herself.
During Mary Warrens questioning, and particularly when she is told to faint, the audience feels acutely her desperation at being requested to perform such an impossible feat. It is easy to understand why she cannot faint without the girls and Danforths pressurising does not help. First he turns around and tells her to Faint! when she is not expecting it, then when she tries to explain why she cannot faint right there and Danforth replies with Why? What is lacking now?
Mary cannot find an answer to this because physically, nothing is lacking. We can see her frustration at not being able to explain how the atmosphere effected how she fainted before and this leads us to sympathise with her, whilst at the same time feeling frustrated towards Danforth and his lack of understanding of human nature. Because of this, even when she accuses Proctor of being the Devils man, we can see that she was under a great deal of pressure, driven to hysterics, and it was the only way out for her.