Arthur Miller’s play All My Sons is the story of the Keller family, which is comprised of the parents—Joe and Kate—and their two sons Chris and Larry. The play is set immediately post-World War II, and Larry has died in the war. Dr. Jim Bayliss is a friend of the Keller family.

During the war, Joe shipped engine parts that he knew were defective. The play, which is a drama, revolves around Joe’s growing realization that his actions contributed to the death of his own son—who essentially killed himself during a mission—and many other young soldiers. Joe eventually realizes that these boys were “all my sons.”

Jim's role in the last act of All My Sons is to serve as a foil to help advance the plot and let the audience understand what Kate is thinking. He also serves as an ethical voice of wisdom and honesty in the scene.

During the dialog between Kate (Mother) and Jim, Mother tells Jim that her son Chris “had an argument with Joe. Then he got in the car and drove away.”

Jim questions her. “What kind of an argument?” he asks. It seems clear to the audience that Jim might suspect the subject of the argument, although he asks, “They argued about Ann?” Then Jim alludes to the true topic and asks whether Joe told Chris the truth.

Mother: (stops rocking) Tell him what?

Jim: Don't be afraid, Kate, I know. I've always known.

Mother: How?

Jim: It occurred to me a long time ago.

Mother: I always had the feeling that in the back of his head, Chris... almost knew. I didn't think it would be such a shock.

Jim: (gets up) Chris would never know how to live with a thing like that. It takes a certain talent...for lying. You have it, and I do. But not him.

[...]

[E]very man does have a star. The star of one's honesty. And you spend your life groping for it, but once it's out it never lights again. I don't think he went very far. He probably just wanted to be alone to watch his star go out.

Jim alludes to Chris's star going out as a way of saying that Chris has always been honest, but if he accepts his father's actions, he will lose his intrinsic honesty and so lose himself. Jim says of himself, “I can't find myself; it's hard sometimes to remember the kind of man I wanted to be. I'm a good husband; Chris is a good son... He'll come back.”

In the final scene, Joe realizes the full impact of his actions. Chris and Kate debate whether it would help anyone if Joe should go to prison. Joe then reads a letter from Larry and says,

Keller: (looking at letter in his hand) Then what is this if it isn't telling me? Sure, he was my son. But I think to him they were all my sons. And I guess they were, I guess they were.

Dr. Jim Bayliss is Joe and Kate Keller’s neighbor in All My Sons. He has lived beside them for years and, along with his wife, Sue, has been a friend to the family. In act 3, Kate has learned that her husband’s neglect at his work most likely caused the death of her husband. Jim, coming home from a house call, checks on his neighbor, who is sitting outside at 2 a.m. He is concerned that she is up so late, and she explains that she’s waiting on Chris to come home: “He had an argument with Joe. Then he got in the car and drove away.” When she won’t open up to him about why they fought, he confesses that he knows the truth about Joe’s actions: “Don't be afraid, Kate, I know. I've always known.”

Jim’s revelation to Kate is important because it shows a difference between Chris and Joe. Kate explains that she always assumes a part of Chris knew, but Jim is quick to explain why that wouldn’t be possible. Jim points out Chris’s honestly and explains that, unlike Joe, Kate, and even Jim, Chris would never have been able to move on from something like this.

Chris would never know how to live with a thing like that. It takes a certain talent ... for lying. You have it, and I do. But not him.

However, Jim also tries to comfort Kate and reminds her how good a son Chris is, telling her she shouldn’t worry because he will return home soon enough. He gives an anecdote to support this, explaining how he once left his wife to research diseases in New Orleans, but he came back to her because she asked him to and because he’s a good husband.

One year I simply took off, went to New Orleans; for two months I lived on bananas and milk and studied a certain disease. And then she came, and she cried. And I went back home with her. And now I live in the usual darkness; I can't find myself; it's hard sometimes to remember the kind of man I wanted to be. I'm a good husband; Chris is a good son ... He'll come back.

Jim Bayliss has an important role in the final act of the play, he explains to Kate that he knew all along that Joe was guilty.  He also tries to reassure Kate that Chris, who has run off since learning that his father truly was guilty, will come back.

Jim reasons it out in this way:

"Oh no he'll come back.  We all come back Kate.  These private little revolutions always die.  The compromise is always made.  In a peculiar way.  Frank is right--every man does have a star.  The star of one's honesty.  And you spend your life groping for it, but once it's out is never lights again.  I don't think he went far.  He probably just wanted to be alone to watch his star go out." (Miller)

What Jim tells Kate, and the reader, is that Chris had his idealism shattered when he discovered his father's deceitful behavior. Chris was incapable of believing that his father would so indifferent to the suffering that was endured in the war, so much so that he would knowingly add to it by shipping faulty cylinder heads that would cause planes to crash.

Jim tells Kate and the reader, that Joe was never really completely safe even in his own backyard from the consequences of his actions, even though he lied over and over again trying to convince himself that he was innocent.

Jim's presence is very reassuring at the end, he brings a realistic perspective to the events that just transpired, the arguments with George, the fight between Chris and Joe.

He is a reasonable, intelligent man, a doctor, so it is easy to believe in his words.  He speaks like a philosopher, but his explanation is very realistic, because he had his personal revolution once too.

Jim makes the reader believe that Joe Keller can be forgiven, but believes that it would be better if Chris did not return to the family home right now.  Chris needs to make his own way, away from Joe and his factory.