This story is largely about a man who causes his old friend to be arrested. It must have been a hard decision for Jimmy to make, but it is interesting to note how O. Henry softens the betrayal in order to keep the reader from forming too unfavorable an opinion of Jimmy the cop.
Bob is treated courteously by the plain clothes man. He is not handcuffed. He is not physically or verbally abused.
The two men started up the street, arm in arm.
An arrest could hardly be more cordial than that. The arresting officer says:
"Going quietly, are you? That's sensible."
Bob offers no resistance. If he were wanted for a serious crime he might try to break away and run for it.
The detective tells him:
"Chicago thinks you may have dropped over our way and wires us she wants to have a chat with you."
Evidently Bob is not wanted for a specific crime. The questioning may be a little more than a "chat," but it seems possible that Bob can talk his way out of it. He is a smooth talker. We the readers do not believe he has committed a really serious crime such as murder or robbery. That is not his style. He must be a con artist. It is interesting to note that the arresting officer refers to Chicago as "she." This, along with the word "chat," suggests something like a social visit. Although the officer is using understatement for humor, it still seems to indicate that Bob is not in really serious trouble.
Furthermore, Bob is prosperous. He can hire a lawyer. He doesn't even have to go to Chicago immediately if he doesn't want to. He would have to be extradited by the state of Illinois. Or else Chicago would have to send a couple of detectives to New York to interview him there. They might decide that it isn't worth the trouble and expense.
So all of this is intended to mitigate Jimmy's betrayal of his old pal. We do not end up disliking Jimmy, although we feel some sympathy for Bob because he liked and trusted Jimmy so much and came such a long distance to meet him after twenty years.
In order to prepare readers for the ending of a story, authors often use foreshadowing. However, as one who often employs the surprise ending, O. Henry does not often give his readers too many hints of things to come in his short story "After Twenty Years." So, the foreshadowing that O. Henry writes is rather subtle; usually the reader only finds it on review.
For instance, in this narrative, there are the two references of diamonds that the man in the doorway owns: a scarfpin and a handsome watch. He has a "little white scar near his right eyebrow, as well. These details suggest the possibility of some one who has an ill-gotten wealth, and it seems a little odd that the policeman does not ask the man for identification. It also seems odd that the policeman chats with the visitor as long as he does.
Less subtle than these details is Bob's reaction to Jimmy when he remarks,
"You've changed lots, Jimmy. I never thought you were so tall by two or three inches."
Of course, he realizes the man is not Jimmy when they stand under the light, and then the reader is fully prepared for the ending
HE MADE IT HAPPILY