Answer:
This complicates the plot because Mary Warren is under pressure and is afraid of being hung up. So she blames John Proctor for studying and worshiping the devil. This creates more tension about witchcraft because there are more and more culprits. I am bound by law, I cannot say it. I must go to Salem, Goody Proctor; I am a court official! In the first act, she wants to tell the truth about dancing in the woods, but Abigail threatened Mary. So Mary is being controlled by Abigail. So we can suggest that Abigail planted the idea of the doll in Mary's head and Mary did the dirty work.