In All My Sons, how does the central character cause other characters to change?
(I think that the answer is Joe Keller.. But I'm not sure.. and I also don't know why he is more important than other characters.)
Joe Keller is the protagonist of All My Sons, and his actions revolving around his decision to harbor a grave secret for many years do have a significant impact on the other characters in the story. For example, his marriage to Kate is strained because she is the only person who knows the truth, Steve Deever is imprisoned as a result of the lie, and Joe's son Chris is shattered at the end of the play when he learns the truth about his father.
However, the central character whose life causes all the characters in the play to change is actually not really a character in the play--the Kellers' other son Larry is a pivotal figure in the play because his disappearance and later the voicing of his ideals are what propel the characters to reconsider their own values. When the play opens, the men are trying to keep Kate from finding out that Larry's memorial tree has been felled by a storm. Kate truly believes that Larry is still alive and that he will one day return. It becomes clear that everyone else has serious doubts about Larry's return, but Kate maintains hope. Near the end of the play, Ann reads a letter to the family that Larry sent to her before he committed suicide. He questions the nature of brotherhood and the responsibility (or lack thereof) that people feel towards each other. After this revelation, the characters begin to reconsider their own values: Joe confesses to his crime, Chris re-visions what he wants from life, and Kate must come to terms with her son's death.
So, Larry is really the character who causes others to change.