The setting of O.Henry's "After Twenty Years" creates a mood of mystery and secrecy. The story takes place at "10 o'clock at night, but chilly gusts of wind with a taste of rain in them had well nigh depeopled the streets." Most people wouldn't be out on a night like this as the quote points out, so the story is immediately shrouded in mystery. Jimmy is checking doors but "suddenly slowed his walk" when he sees a man leaning against the door of the hardware store. The fact that Jimmy "suddenly slowed" indicates surprise and that he is seeing something out of the ordinary, adding tension to the story.
Silky Bob lights his cigar revealing "a pale, square-jawed face with keen eyes and a little white scar near his right eyebrow." This description clearly creates mystery and suspense, each detail slowly revealing a face that has a scar. Most people don't have scars on their faces, so this specific detail increases suspense.
O. Henry is sympathetic toward both characters, but especially toward Jimmy the police officer. His description of Jimmy at the beginning of the story indicates a man who is responsible and takes his duties seriously. He checks every business door as he completes his rounds with a "watchful eye." O. Henry also describes him as a "fine guardian of the peace." This portrayal indicates a respect for this character's dedication, even on such an inhospitable evening.
O.Henry's depiction of Bob is not as complimentary, although he does highlight Bob's loyalty. Most men, especially if involved in crime, would not keep such an appointment after twenty years. However, this also points to Bob's arrogance which ultimately sets up the surprise ending.