Both the reader and "Silky" Bob are doubly surprised by the ending of the story. Both believe that the plain clothes detective who invites Bob to come along and have a drink with him is Jimmy Wells-- although the reader may detect a clue to the detective's identity and purpose when Bob says:
"You've changed lots, Jimmy. I never thought you were so tall by two or three inches."
Bob does not discover the truth until they reach the brightly lighted corner. Then he says:
"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."
Then the arresting officer hands him a note, and Bob and the reader have a second surprise. The uniformed cop Bob had been talking to a little more than twenty minutes earlier had been his old friend. The note reads:
"Bob: I was at the appointed place on time. When you struck the match to light your cigar I saw it was the face of the man wanted in Chicago. Somehow I couldn't do it myself, so I went around and got a plain clothes man to do the job. JIMMY."
When Jimmy was about to leave, he said:
"I'll be on my way. Hope your friend comes around all right. Going to call time on him sharp?"
"I should say not. . . . I'll give him half an hour at least."
Jimmy has only a half-hour in which to go "around" to his station and find another officer to impersonate him. The one he gets is described as "a tall man in a long overcoat, with collar turned up to his ears." He is taller than Jimmy and has an entirely different kind of nose, but he is the best Jimmy can get on such short notice.
According to O. Henry's description, the tall man "hurried across from the opposite side of the street." This shows time is of the essence. Jimmy had to get to his station, explain his problem, and find someone to impersonate him. Then his substitute had to get to the hardware store before Bob left. That is why O. Henry uses the word "hurried."
Bob has a clue that the plain clothes man is not Jimmy when he notices that he is two or three inches taller than his old friend. Was there any clue that the uniformed officer he was talking to before was in fact Jimmy Wells? It is pure coincidence that Jimmy has been assigned to a beat that includes the place where he had agreed to meet Bob twenty years earlier. Jimmy is a bit early for their appointment, so he is doing what he always does on patrol, including trying every shop door to see that it is securely locked. It is this typical policeman's behavior that keeps the reader from suspecting that the cop might be Jimmy Wells himself.
Jimmy doesn't realize that Bob is the man wanted in Chicago until:
The man in the doorway struck a match and lit his cigar. The light showed a pale, square-jawed face with keen eyes, and a little white scar near his right eyebrow.
This is a very good touch. The flame will naturally light up Bob's face but not provide any illumination for Jimmy's. Another nice touch is that Jimmy is a cop in uniform. Bob would never suspect that his old friend would have joined the force. Jimmy lets Bob do most of the talking. He doesn't want his voice to give him away. Neither the reader nor "Silky" Bob gets a clue that Jimmy is the policeman, although there is a definite clue that the plain clothes detective is not Jimmy Wells.
"After Twenty Years" is a very short story but covers twenty years, two mistaken identities, and two surprise endings. This shows O. Henry's ingenuity at its sharpest.