Abigail Williams
Possibly the main character in the play and certainly the chief villain, Abigail is quite a multifaceted character. Delve into her motivation, her strengths, her weaknesses. Does she really believe in witchcraft? Does she really believe so many women have made a pact with the Devil?

Most of your information will come from a close reading of the actual play. Pay close attention to Abigail's scenes in Act 1 and Act 3.
You might also want to check out the following books:
Bloom,
Harold. Bloom's Reviews: Arthur Miller's The Crucible.
New York: Chelsea House Publishing, 1998.
Johnson,
Claudia Durst, and Vernon E. Johnson. Understanding The Crucible.
New York: Greenwood Publishing, 1998
Martine,
James J. The Crucible: Politics, Property, and Pretense.
New York: Twayne Publishers, 1993.