In Act III Mary Warren is brought to court by John Proctor to tell the truth for once. John Proctor's wife has been arrested after false accusations and he believes Mary Warren is the only person who can save her. While it sounds like a good plan, the courts are not quick to believe her.
Danforth points out that when Mary Warren was "afflicted" by the accused witches, she would faint and become cold. Hathorne tells her and Proctor
Then can she pretend ... So let her turn herself cold now, let her pretend she is attacked now, let her faint.
Mary Warren explains that before she could faint because the girls around her were fainting. She found herself getting caught up in the excitement. She even points out that Danforth and Hathorne seemed to eagerly believe the girls, so she began to believe them too.