How does O.Henry create a balance between foreshadowing and surprise in "After Twenty Years."?

In "After Twenty Years," O. Henry molds his main characters with a certain anonymity that provides some foreshadowing while at the same time providing for the surprise ending.
Foreshadowing
In the exposition of the story in which two old friends have agreed to meet twenty years after their parting ways, the first character who is introduced is simply referred to as the "policeman." Also, since this policeman, an officer of the law who patrols a certain neighborhood, is a familiar personage to readers of O. Henry's time, there is nothing about him which would lead these readers to think that he has a primary role in the narrative. His routine questioning of the man waiting for his old friend in the doorway arouses no suspicions, either, but it does provide subtle foreshadowing as the man's lighting of his cigar reveals a "little white scar near his right eyebrow" and a large diamond as a scarfpin. Another very subtle hint of what is to come are some words said by this stranger:

"We figured that in twenty years each of us ought to have our destiny worked out and our fortunes made, whatever they were going to be."

Surprise Ending
Since the policeman on the beat seems insignificant, with the later appearance of "a tall man in a long overcoat" who goes directly to the man in the doorway, readers may well assume that he is Jimmy, the stranger's old friend for whom he has been waiting, especially because he asks, "Is that you, Bob?" But, when this tall man steps under the bright lights of a drug store as the two walk along arm-in-arm, the old friend suddenly stops, removes his arm, and says, "You're not Jimmy Wells," the narrative is then set up for the surprise ending. For, Patrolman Jimmy Wells has sent a plainclothes policeman to meet 'Silky Bob' and to give Jimmy's letter to his old friend, explaining why he has not met him.