"After Twenty Years" is one of O. Henry's shortest stories. It is remarkably effective for such a compact story. It starts close to the climax. Bob tells Jimmy, whom he doesn't recognize, that it is three minutes to ten, the time of their appointment.
About twenty minutes he 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.
At the end the plain clothes man tells Bob:
"You've been under arrest for ten minutes, 'Silky' Bob.
So the time covered is only about thirty-five minutes--five minutes for the cop to notice Bob in the doorway and chat, twenty minutes for Bob to wait, and ten minutes with the man who arrests him. This is a remarkable piece of literary craftsmanship, especially since twenty years is covered retrospectively.
The plain clothes man whom Jimmy, the patrolman, summoned to make the arrest, never actually says he is Jimmy Wells. In fact, he says as little as possible before he makes the arrest, because his voice might give him away. Bob hasn't talked to Wells for twenty years, so he might not expect his voice to be the same as when he was twenty.
The arresting officer has turned up his overcoat collar to conceal much of his face. This does not arouse Bob's suspicions because:
There was now a fine, cold drizzle falling, and the wind had risen from its uncetain puffs into a steady blow. The few foot passengers astir in that quarter hurried dismally and silently along with coat collars turned high and pocketed hands.
Furthermore, the story starts off in darkness, although it ends under bright lights. O. Henry does not actually say it is quite dark where Bob is waiting. That would be somewhat inartistic. It would alert the reader that darkness was important--that the story might hinge on mistaken identities. But the author specifies it is ten o'clock and
The vicinity was one that kept early hours. Now and then you might see the lights of a cigar store or of an all-night lunch counter; but the majority of the doors belonged to business places that had long since been closed.
It is entirely plausible to the reader that Bob would not recognize the plain clothes man as an impostor because of the darkness, the fact that he had not seen Jimmy in twenty years, and the fact that the impostor kept his overcoat collars turned up to conceal both sides of his face. He is experienced and savvy enough to let Bob do most of the talking--which is easy to do, considering Bob's expansive character.
The man from the West, his egotism enlarged by success, was beginning to outline the history of his career. The other, submerged in his overcoat, listened with interest.
Bob is no doubt giving away at least some information about his illegal activities in the West. He is the type of man who wants to impress others, which explains his diamond scarf pin and ornate watch with the lid set with small diamonds. Significantly, the arresting officer is "submerged" in his overcoat. He "listened with interest" because this was a way to keep Bob talking rather than asking him questions, and also because this was a good way to collect incriminating information.
When they come to the corner "brilliant with electric lights," the plain clothes man naturally has to disclose his true identity; but he has played his role perfectly up to this point--and O. Henry has succeeded in achieving the surprise ending which is essential to his story.