Final answer:
Mary Warren's behavior changes in Act II of The Crucible due to a personal conflict, leading her to become more secretive and deceptive.
Step-by-step explanation:
In Act II of The Crucible, Mary Warren's behavior has changed compared to Act I. She is more secretive and deceptive due to a personal conflict. In Act I, Mary is easily influenced by the other girls and is afraid to go against them. However, in Act II, she becomes caught between her loyalty to the Proctors and her fear of the consequences if she reveals the truth about the girls' pretense. This conflict leads her to engage in deceitful behavior, such as lying to protect herself and John Proctor.