What is the primary center of tension in the encounter between John Proctor and Mary Warren? Why does Mary begin to challenge Proctor's...

In Act Two, when Mary Warren returns home to the Proctors' house, the primary reason for the tension between John and Mary is that he had forbidden her to go into Salem.  She had disobeyed his order, as well as his wife's, and gone to town anyway.  As Elizabeth explains, Mary Warren is no longer like a mouse; "she raises up her chin like the daughter of a prince and says [...], 'I must go to Salem, Good Proctor; I am an official of the court!'"  Thus, Mary now feels as though she can defy Proctor's authority because she believes that she is answering to a higher authority, a more important authority than her employer: the court.  Mary explains that, although there were fourteen accused before, there are now thirty-nine accused, and she is "amazed" that John is unable to see "what weighty work [they] do" in the courts. 

This interaction reveals how dramatically and quickly the court's authority has increased as well as the importance it places on the words of the girls.  Elizabeth explains that judges have been brought in from Boston, and presiding over all is the Deputy Governor of the colony.  The court's authority is so far-reaching that it seems to overtake logic and reason, and the (lying) girls have gained a measure of this authority for themselves.