Show that the policeman's impressiveness was habitual and not for show?
O. Henry does a masterful job of introducing one of the principal characters, Jimmy Wells, and concealing that character's identity at the same time. The author explains how the policeman's impressiveness was habitual and not for show when he writes:
Trying doors as he went, twirling his club with many intricate and artful movements, turning now and then to cast his watchful eye adown the pacific thoroughfare, the officer, with his stalwart form and slight swagger, made a fine picture of a guardian of the peace.
O. Henry's readers would have been quite familiar with the appearance and behavior of the typical New York beat cop, and they would have been beguiled into assuming that this was just one of them and not the man Bob was waiting for in the dark doorway of the hardware store. Even when the unnamed policeman stands there talking to Bob, the reader does not suspect that this might be the very person Bob has been waiting for and looking forward to meeting again after twenty years. If Bob doesn't recognize him as Jimmy Wells, why should the reader recognize him?
O. Henry wanted to introduce the policeman and at the same time show that he had been "a guardian of the peace" for a long time. The most telling words in O. Henry's description of the policeman's habitual impressiveness are:
... twirling his club with many intricate and artful movements,
It obviously takes time on the job to develop such habits and such "intricate and artful movements." The fact that he keeps trying doors along the way in order to make sure they are safely locked completely deceives the reader, who assumes that this policeman is simply patrolling his beat and therefore could not be there for another purpose. Jimmy has a few more minutes before his appointment with Bob, which he fully intends to keep. It is natural that he would be following his usual routine to fill up the time and attend to his duties. When he does go to meet Bob at the doorway of the hardware store, it is natural for Bob, and the reader, to assume that the policeman is only there to question a man who looks a little bit suspicious standing in a darkened doorway. It turns out that Bob has backed into the doorway to light his cigar in the blustery weather. Bob, characteristically, does most of the talking and doesn't give Jimmy much of a chance to identify himself before he lights his cigar and reveals the face of a man Jimmy knows to be wanted by the Chicago police. Jimmy can see Bob's face very clearly by the light of the match, but Bob can only see the silhouette of a typical cop in uniform and cannot make out his face. Chances are Bob might not even recognize Jimmy after twenty years even if he could see his face. But the same match that lights up Bob's face would naturally blind him to the other man's face until 'Silky' Bob has lighted his cigar and blown the match out, leaving them both in the dark.