In Act III, Parris, Danforth, and Hathorne ask Mary Warren to faint (or pretend to faint believably). John Proctor has brought her to court to tell the truth, that the girls are only pretending to see signs of witchcraft, and that they have been lying this whole time
In order to prove that she was only pretending in court, Hathorne asks, "Then can she pretend to faint now?" Parris claims that there are no spirits attacking her now, so if there really were no spirits attacking her in court, she ought to be able to replicate her previous behavior. Her inability to faint now, to make herself turn cold as she did before, seems to confirm that she was, in fact, being attacked by spirits before and that this is what caused her to faint then. Mary tries to explain what was different before, but she is unable to do so; Danforth questions Abigail about Mary's claims nonetheless, but when Abigail threatens him, he relents.
Mary Warren has admitted to faking the sighting of the devil, the fainting, the chills, the shaking. John Proctor brings her into the court room to prove the girls are lying, but Mary is unable to fake seeing the devil, perhaps because Abigail was their leader and was able to "act it" better than the other girls, and lead them into believing.