Act 1 is a large act... let's look at each character individually:
ABIGAIL WILLIAMS: Miller suggests that Abigail is cunning, sneaky, manipulative and selfish throughout his stage directions of her relationships with the other girls, as well as her relationship with John Proctor. He does this through directions like "grasping his hand before he can release her" and "now beginning to anger - she can't believe it". One of my favorite directions is "winningly she comes a little closer, with a confidential, wicked air." We see this parallel her dialogue with John Proctor as she confidently suggests they get back together.
ANN PUTNAM: Miller paints Ann much more directly through her own words as a woman in support of her husband and as one who is very hopeful that there is indeed witchcraft in the Parris household. However, one striking stage direction has her speaking "with a growing edge of sarcasm". Sarcasm is not becoming of a good Christian Puritan woman. However, Mrs. Putnam's words mock Rebecca Nurse's great bounty of children while insisting that her own failure to multiply weighs mightily on her.
MERCY LEWIS: Upon entry, Mercy is described in stage directions as "the Putnams' servant, a fat, sly, merciless girl of eighteen." Her image immediately portrays itself as when the girls are alone she is ready to beat the pretend out of Betty.
Stage directions are, essentially, a playwright's instructions on how a play should be performed. They include stipulations on how the actors should move, speak, and the tone they should adopt. They can also include directives on the lighting and decor on the stage to set a particular scene. The stage directions, therefore, enhance the characterization and add to the mood.
Arthur Miller's stage directions in The Crucible tell us more about the personalities of the three characters you mention and, therefore, create an expectation of how they may respond or behave in a variety of settings.
At the beginning of Act 1, Miller says the following about Abigail Williams when she appears:
Abigail Williams, seventeen, enters—a strikingly beautiful girl, an orphan, with an endless capacity for dissembling. Now she is all worry and apprehension and propriety.
The description makes it clear that Abigail has an extensive ability to hide what she feels and tends to put on an act. She, therefore, would find it easy to deceive and mislead. The fact that she is beautiful probably has helped and will help her in whatever ventures she aspires to since it is easy to believe someone who looks pleasant. Furthermore, the fact that she is an orphan suggests that others will readily take pity on her. At this moment, Abigail is full of concern and anxiety. She is well behaved and respectful.
Miller also states that Abigail is "quavering," and that she "lowers her eyes," which means that she is trembling with either anxiety or emotion. The fact that she looks downwards suggests that she is respectful and somewhat ashamed at this point.
True to her character, Abigail acts as if she's innocent at this point and seemingly terrified when her uncle suggests that he saw someone naked in the woods. Her innocence soon turns to resentment, though, when the Reverend asks her about her discharge from the Proctor's service. She becomes angry when her uncle speaks about her not being hired by anyone since her dismissal. These actions suggest that Abigail is short-tempered and volatile.
Miller further presents Abigail as an afraid young girl in the early parts of Act One. He mentions that she is filled with "hushed trepidation," and that she goes to Betty and speaks to her "with fear in her voice." It is apparent, however, that she is violent and aggressive. When Betty cries out she, for example, "smashes her across the face" and later "roughly sits her up." Her actions are supported by what she says. She threatens the girls with a "pointy reckoning" if they should speak about what happened in the forest.
In her encounter with John Proctor, we see how Abigail is filled with desire. She knows that she has something on him and her approach is "confidential and wicked," as if she expects him to react positively to her. She utters a "thrill of expectant laughter, clearly" and she is "feverishly looking into his eyes." Her desire is "concentrated," and she taunts him. When he refuses her advances, she becomes angry and vents her bitterness towards Elizabeth, John's wife.
At the end of the Act, the stage directions depict how opportunistic Abigail is and what an actor she can be. She "instantly points at Tituba" and starts blaming her for what the girls did. When Tituba starts "confessing," she follows suit and starts naming so-called witches. She is "enraptured, as though in a pearly light." Her, Tituba, and Betty's actions are what initiate the devastating and tragic events in the play.
The stage directions make it quite evident that Mrs. Putnam is absolutely convinced about what she believes and that she will challenge anyone who questions her conviction. She states "with vicious certainty" that the Devil has touched their children. We see that she cannot believe that Rebecca Nurse has goodness in her because she is "astonished" when Rebecca calms Betty. She is clearly resentful of Rebecca and is obsessed with the idea that she has lost so many children. She expresses "with a growing edge of sarcasm" the suggestion by Rebecca that it is God's work that they should look to for answers about the girls.
Mrs. Putnam's superstitious nature is further confirmed when she is "frightened" and "speaks very softly" about having sent her daughter to Tituba to conjure the spirits of her dead children. Stage directions state that "her voice breaks" when she speaks about her seven dead children, which makes it clear that it is a truly sore point for her. In the end, it is her superstition and bitterness that lead to Rebecca's arrest and incarceration.
The stage directions suggest that Mercy Lewis is not a pleasant person at all.
Enter Mercy Lewis, the Putnams’ servant, a fat, sly, merciless girl of eighteen.
Ironically, the directions state that she is the opposite of what her name suggests. The descriptors indicate that she is devious and lacks compassion. This is proven when she declares how she treated Ruth when she smacked her. She wants to do the same to Betty, but Abigail holds her back. The stage directions indicate that she is also frightened when Abigail tells her that Parris saw her naked. Her actions also show that she is aggressive and vindictive. She points at Mary Warren and later moves "menacingly toward Mary." This suggests that she is just as nasty as Abigail and will hurt others to save herself. Mercy is also attracted to John Proctor for the directions state that she was "both afraid of him and strangely titillated." She must be aware of what had transpired between John and Abigail.
All in all, these three characters are probably responsible for most of the accusations and arrests for witchcraft in the village. They were all either embittered, vengeful, merciless, or all three.