Why do Bob's hands began to tremble when starts to read the note in "After Twenty Years"?
Bob is called 'Silky' Bob only one time in the story. This is when he is told by the plain clothes officer that he is under arrest. But Bob's behavior up to that point has been cool and confident. Although he is now a stranger in New York, although he must know he is wanted by the law in various places, and although he realizes he may look a little suspicious standing in a doorway in front of a locked and darkened hardware store, Bob is completely at ease when he is approached by the uniformed policeman. When we learn that he has the nickname of 'Silky' Bob, this only adds to our picture of a successful middle-aged man who is a smooth talker and who likes expensive things, including silk shirts, silk ties, silk handkerchiefs, silk scarves, silk stockings, and silk underwear.
O. Henry specifies that 'Silky' Bob's hand trembled a little mainly because the author wants to demonstrate that Bob loses a little of his cool. Bob's trembling hand provides a visual contrast between his cool facade and his strong feelings of disappointment, apprehension, and especially chagrin resulting from the fact that he--a man who prides himself on succeeding by living by his wits--has been completely outwitted. He wasn't even smart enough to realize that the uniformed cop he had been boasting to in front of the hardware store was actually his old friend Jimmy Wells. He even provided Jimmy with positive identification by reminiscing about their old friendship and their promise to meet at that location after twenty years. Then he remembers lighting a cigar and giving Jimmy a good long look at his face.
O. Henry does a masterful job of covering a time span of twenty years in just a few pages, while using a single setting of about one block in New York City.
As he stands in the doorway, waiting to reunite with an old friend twenty years after they have last been together, the man with the diamond cravate pin appears somewhat anxious. He tells the beat patrolman that he and the friend of his youth agreed one night years ago to meet again in precisely twenty years,
...no matter what our conditions might be or from what distance we might have to come. We figured that in twenty years we ought to have our destiny worked out....
Further, he tells the policeman that he has come a thousand miles to see his old friend. Later, when a man finally comes, 'Silky Bob' thinks it is his old friend Jimmy until they pass under a street lamp and he identifies the man's facial features. The plain clothes policeman informs Bob he is under arrest and hands him a note from Patrolman West. As he reads it, his hands tremble because Jimmy is still a friend, but he is also a policeman, who feels he must do his duty. Out of respect for their friendship, Jimmy has another policeman deliver his message; nevertheless, Bob is bound for prison. He is a criminal, and his old friend is a policeman.