Final answer:
Mary Warren cannot faint when ordered to in court because the mass hysteria and social pressure that enabled her previous actions are absent. This reveals the psychological manipulation at the heart of the Salem witch trials, undermining the belief in actual supernatural forces at work.
Step-by-step explanation:
In Act Three of The Crucible, Mary Warren cannot faint on command when asked in court because her actions are not driven by actual supernatural forces but by social pressure and hysteria. When previously Mary Warren had fainted, she was caught up in the contagious hysteria of the courtroom and the power of suggestion from other girls, which is notably absent in a controlled environment where she is under intense scrutiny. This inability to faint reveals the psychological aspect of the witch trials, and how the power of suggestion and group dynamics play a critical role in the accusers' behavior.
The pressure Mary feels in the courtroom, with all eyes on her awaiting her demonstration of her 'fainting' ability, appears quite different from the collective frenzy experienced with the other girls. This moment is a clear indication that the witch trials are based more on fear, suggestion, and collective mania than on any actual evidence of witchcraft.