How does Big Mama's character evolve in Act 3? Does she go back to her former self in the end?
Big Mama's character evolves in Act III as she embodies the Christian charity and selflessness that she has sought to impart to her children throughout the drama. When she realizes that her husband really is going to die, her demeanor of selflessness cuts through the "mendacity" that Big Daddy has raged against throughout the drama.
Her character evolves from being a good natured, humorless, and light figure to one who possesses a sincere amount of depth and sensitivity. She does not go back to her former self at the end of the drama. Rather, she recognizes that something profound is upon her when confronting the death of a man that has been her life for so long. The jokes are gone. In its place is an embrace of her responsibility as one who cares and loves another. She is saddened by the reality of the situation, and this is what makes her the selfless image of Christian service that she has emphasized to everyone around her for so long. With all that surrounds the characters, she transcends her former self, rising above it to become more than the sum total of the contingencies that are around her.