The girls at the center of the trial quickly become taken with their own importance. They have found that they wield a strange power, one that they could not ever have had otherwise. They are celebrities of a sort because of the trials.
I do not know off-hand how true this is. I would like to think this is a fictional element Miller used to create a parallel between the Salem Trials and the Senate hearings run by Sen. McCarthy. People who were willing to name communists during the Red Scare wielded a similar power and that led to more widespread panic. We must always remember that this was Miller's real message.
Elizabeth's name was mentioned in court and Mary Warren defended her saying she was a good woman not involved in witchcraft. Mary Warren could easily turn to the court and speak otherwise if Elizabeth doesn't speak kindly to her.
Mary Warren feels that she has gained power due to her ability to testify in court. She works for Elizabeth, but now feels that she should gain some respect from her employer, and that she could even be a threat to her.