The second act of the play serves to do at least two things. The tension of the play is heightened as the Proctors become directly involved in the Salem trials and the moral weight of the narrative is connected to multiple characters. By the end of the second act, nearly every character is morally involved, implicated and responsible in some way for the proceedings of the court.
Elizabeth Proctor is formally accused of witchcraft in this act (through the subtle genius and acting of Abigail). This places the Proctors into the action of the trials, absorbing the entire narrative into what had been a regrettable fraud being perpetrated on other people in the town.
At this point, the fate of the Proctors is aligned with the fate of all the others who have been accused.
Proctor's guilt regarding his infidelity is discussed in this act as is Elizabeth's implied guilt in her cold, unforgiving stance. Mary Warren is implicated as a tool of Abigail's plot and is now morally enmeshed in Elizabeth's fate. Hale is given good evidence that the trials are fraudulent and so becomes implicated as well.
At the end of the act, the conflicts of the play are clear and tense. Elizabeth must be saved along with the others who stand accused. Proctor must act. Hale faces a moral dilemma and Mary Warren does as well.