There's not much foreshadowing in this story, but O'Henry drops a few hints that Bob might be a criminal, such as mentioning the scar on his face and the diamond studded watchcase he carries. O'Henry also foreshadows that the person who talks to Bob at the end is not Jimmy when Bob says:

You've changed lots, Jimmy. I never thought you were so tall by two or three inches.

The point of view is third-person omniscient, but the narrator chooses to withhold certain facts from the reader until the end, which adds to the surprise. The most important withheld fact is that Jimmy Wells is the officer who initially approached Bob.

O'Henry is famous for his use of situational irony, which is when events work out differently than the characters expected. The chief irony is that a reunion with his best chum that Bob looks forward to as a happy time becomes the occasion of his arrest. Another irony is that Bob looks down on Jimmy as a plodder and sees himself as a success story, but by the end of the tale, the tables have turned.

The suspense comes in wondering whether Jimmy will show up and what he will be like if he does.

The master of the ironic reversal, O.Henry creates in "After Twenty Years" a narrative that is both suspenseful and touching by utilizing other literary techniques, as well.

•  Foreshadowing

In the beginning of the story, the scene in which a policeman sees a man leaning in a darkened doorway, and the man in this doorway quickly explains his purpose for standing there, is an example of foreshadowing, or hinting at things to come in the narrative. Specifically, as this stranger explains who he is and why he is there, he lights his cigar, revealing a small scar under his eyebrow and a large diamond "scarfpin." These details of how the man in the doorway looks suggest that there may be a history behind this man.

Also, the fact that the policeman asks the man in the doorway if he is going "to call time on him sharply" ("After Twenty Years") hints that for some reason the policeman wants to know how long this man will remain there.

• Point of View

Point of view is closely connected to the suspense of the story because, while there is technically a third person objective narrator, the scene at the doorway is presented in the manner of an officer of the law who talks to a stranger, questions him, and then reports what occurs without any emotional or artistic interpretation. Later, in the scene in which Bob thinks that Jimmy meets him, the point of view remains third person narrator, but the style moves to the perspective of Bob. With this very limited point of view, suspense is effectively increased.

About twenty minutes he [Bob] waited, and then a tall man in a long overcoat, with collar turned up to his ears, hurried across from the opposite side of the street. He went directly to the waiting man. ("After Twenty Years")

Then, as Bob and the other walk together, the tall man walks under a street lamp, and Bob realizes that this man cannot be Jimmy. He is angered and fearful:

"You're not Jimmy Wells,...Twenty years is a long time, but not long enough to change a man's nose from a Roman to a pug." ("After Twenty Years")

So, to Bob's surprise, he is arrested. Then Bob is handed a note that he holds with a steady hand that starts to tremble as reads what his old friend Jimmy has written.

• Irony

The ironic reversal that Bob has become a criminal wanted in Chicago while his former best friend is a New York City policeman is given a certain poignancy because Jimmy does not have the heart to arrest his friend who has come hundreds of miles to reunite with him. 

• Suspense

Suspense in O. Henry's story is generated as the reader wonders if Jimmy will arrive and meet his friend Bob after twenty years. Then, when a man who is taller with a different nose from that of Jimmy appears, there is more suspense created.

Of course, the surprise ending that reveals the tenderness of a police officer who performs his duty is a result of the suspense created throughout the narrative. For, the identity of Jimmy Wells is not revealed until the very end of his letter.