When Mary Warren appears in court, Parris is concerned. Why?

When Mary Warren enters the court room in Act Three, she is walking with John Proctor. This is the main reason Parris is uneasy when she enters. He and Proctor have always had a contentious relationship. In Act One, they argue over Parris's rhetoric and Proctor says that if there is a party against Parris, he would join it.

When Proctor enters with Mary, Parris immediately expresses his mistrust of Proctor, "Beware this man, Your...

When Mary Warren enters the court room in Act Three, she is walking with John Proctor. This is the main reason Parris is uneasy when she enters. He and Proctor have always had a contentious relationship. In Act One, they argue over Parris's rhetoric and Proctor says that if there is a party against Parris, he would join it.

When Proctor enters with Mary, Parris immediately expresses his mistrust of Proctor, "Beware this man, Your Excellency, this man is mischief." Parris knows that Proctor is there to stand up for his wife, Elizabeth. John Proctor has brought Mary Warren to refute the girls' claims about witchcraft. Like Danforth, Parris does not want the court to be challenged. He thinks that if the court's credibility is challenged, then his credibility will be challenged and questioned as well. Parris is more interested in protecting his own reputation than he is in uncovering the truth. 

Parris also points out that John has not been coming to church. John responds that it has nothing to do with losing his spiritual path. Rather, John simply doesn't care or have any respect for Parris. So, Parris is concerned because he knows John is his enemy and he is afraid that John will use Mary to challenge the court's accusations. Parris will do nearly anything to protect his and the court's reputation.