What changes occur in Salem between Acts 1 and 2 in The Crucible?

In Act II of The Crucible, we learn that the town of Salem has changed quickly in the eight days since Act I through the words of Mary Warren and Elizabeth Proctor.  Mary Warren comes from town, and it is through her dialogue, as well as previous discussions she has had with Elizabeth, that we learn about the town's changes.

Since the accusations of witchcraft began in Parris' house, a court has been formed to seek out and try witches.  While John, knowing that the girls are faking, believes nothing will come of it, Elizabeth knows better. She has spoken with Mary Warren and tells her husband, "There be fourteen people in the jail now, she says. And they’ll be tried, and the court have power to hang them too."  Elizabeth reports that Abigail seems to be in the center of the storm as people look to her to place blame on suspected witches in the town.

The normally levelheaded town has been quickly caught up in the hysteria.  When Mary Warren returns, she reports that Goody Osburn will hang for her supposed crime.  Since she will not confess, the court has decided that she is guilty.   This only foreshadows darker times ahead for the people of Salem.

What changes occur in plot, character, or tone in Act 3 of The Crucible?

There are a lot of major developments in Act III of The Crucible.  The hysteria and accusations have reached a fever pitch, the tone becomes even more serious and, particularly toward the end of the act, even infuriating.  The act begins on a somewhat hopeful note, as Giles Corey says that he has evidence that will show that Putnam is only trying to gain more land.  Further, Proctor appears with Mary Warren, who is ready to confess that she and the other girls are just pretending.  Parris grows nervous, saying that Giles and Proctor are trying to overthrow the court, and Danforth questions Proctor about only attending church once a month and plowing on Sundays.  Initially it appears that the hysteria may come to an end.  It is also revealed that Elizabeth will not be executed for awhile, because she is pregnant.  Proctor also submits a deposition that ninety-one land owners signed, testifying to the good character of Elizabeth, Martha Corey, and Rebecca Nurse.  Things are looking positive; however, Parris wants to question all those who signed the deposition, because he says they are trying to undermine the court.

Hale finally steps up and challenges Parris, and Putnam is then brought in to answer Giles's accusations that Putnam is only out for land, but Giles will not reveal the name of the person who gave him the information, because he doesn't want the person to be another victim of Putnam's deceit.  Unfortunately, things begin to shift back.  Because he won't give the name, Giles is arrested for contempt.  Danforth then sends for Abigail, who denies Mary's claims, but Mary continues to say that it is all an act.  When Hathorne insists she act for them, she can't do it.   At this time Abigail begins to act as if Mary is bewitching her, and the other girls start to do the same.

In order to put an end to everything, Proctor confesses his affair and explains that Abigail is trying to get Elizabeth executed, so Abigail can take Elizabeth's place as his wife.   Proctor says that Elizabeth will attest to the affair, and that she would never lie.  Elizabeth is brought in, but she is not allowed to look at Proctor.  Unfortunately, when she is questioned, she lies in order to protect Proctor's integrity.  She says that she only suspected that there was an affair, and she fired Abigail in anger. Proctor cries out that he confessed, but it is too late for her to change her story.  Hale asks Danforth to reconsider, again taking a firm stand against Abigail and the hysteria.

Abigail and the girls begin to act out even more, saying Mary is hurting them.  They repeat Mary's pleas for them to stop acting.  Things grow even more chaotic, and suddenly Mary begins screaming too.  As Proctor tries to calm her down, she screams that he is the devil's man and says he tried to lead her to work with the devil as well.  Despite Hale's protest, Danforth orders Proctor to be arrested.  Hale denounces the proceedings and says he is quitting the court.

Act 3 is a roller coaster of emotion.  The tone shifts repeatedly, and characters further establish their integrity or their deception.  There are a number of climactic moments, from Elizabeth's first and only lie, to the repeated shifts in power and control.  By the end of the act, the goodness of John, Elizabeth, and Hale are cemented, as are the sins of Putnam, Parris, and Abigail.