It has been a long time since I first read the story, but I think it did more than surprise me. That is putting it mildly! I naturally identified with Peyton Farquhar. I shared his feeling of hope and possible salvation when he imagined that the hanging-rope had broken and he was being carried away from the Owl Creek Bridge by the cold, rushing water. I shared his relief when he seemed to have made it safely to the shore and was running for his life. I shared his growing feelings of confidence as he made it farther and farther away from that awful bridge and closer and closer to his home and beautiful wife.
And then when the slack in the hanging-rope ran out and his neck was broken--I felt that "stunning blow upon the back of the neck" and saw that "blinding white light...with a sound like the shock of a cannon," which Ambrose intended his readers to feel, see, and hear, and which is what every hanged person must feel, see, and hear when they fall as far as the slack in the rope will permit.
In reading the story over again, I can only experience those sensations faintly. I guess they can only hang you once.