At the beginning of the play, Mary Warren is characterized as a bit of a frantic, whiney, cowardly girl who doesn't have much backbone. As the girls gather around Betty and are talking about what to do, Mary Warren comes in, freaking out. She is super worried that they are going to get in trouble for the dancing they did the previous night. She wants them all to 'fess up to the dancing so that she doesn't get into trouble. She adds, self-righteously, "I never done none of it, Abby. I only looked!" First of all, while the dancing was going on, Mercy was to much of a "goody-goody" if you will, to join into the dancing. Then she is the first to wuss out and want to confess their crimes. At least, this is the impression that Miller puts across. Abby is seen as the popular ring-leader and Mary is the more annoying pansy of the group. Abby summarizes it well: "Oh, you're a great one for lookin', aren't you Mary Warren? What a grand peeping courage you have!"
Later however, in Act Two, we see Mary grow a bit of a backbone. She is "an official of the court now", all high on her major role in the accusations of witchcraft. For the first time, probably, she feels accepted, noticed, and important. People listen to her. She even confidently declares to John Proctor, "I'll not stand whipping any more" in the face of threats that used to send her cowering. She demands that he "speak civilly" to her. This change comes from her acceptance in Abby's "clan", in a sense of righteous duty in the courts, and in her very word being the force that impacts so many lives there.
But then, when Proctor wants her to face Abby, she turns into a quivering mass of fear again, trembling at the thought of turning on the girls. She knows what power they have. When she gets to the courts she tries to be strong, but eventually turns on Proctor. It seems that the only time that she can be strong and confident is in the arms of Abby and her clan, and unfortunately, it has devastating results.