So, who plays the killer? (Familiar with faces of Korean extras? ;)

We do see the killer's face, of course - moments after his dark silhouette rises in the grass during the rain, he emerges from it and attacks:
http://img855.imageshack.us/img855/5704/snap1v.jpg

Now, what I wonder is if this was just a single appearance, by anyone (a crewmember, an extra...), or if that actor/extra/crewmember was also placed anywhere else in the film.

I know I would have put the character somewhere else. Somewhere in the background of another scene - most likely, I would have him somewhere among the workers in the factory, for a few seconds. Perhaps as that worker who seems to be completely unnoticeable and is just standing there. Or the one who seems shocked when the cops attack the innocent guy...
http://img269.imageshack.us/img269/9959/snap3oh.jpg

Many seem to believe that the killer is the repairman in the cellar:
http://img705.imageshack.us/img705/4710/snap2ym.jpg

Personally, I don't think so: he is too obvious and too visible... suggesting that the repairman is the murderer would be too vulgar, too much "on the nose". I think this particular character serves to show and stress the indifference to police brutality and abuse of powers, rather than to be used as something as obvious as the old "he's always been right under our noses" routine.

As I said, I know I would have placed the person who played the killer in the attack scene somewhere again for a few frames. Somewhere where he'd be barely visible, perhaps even blurred, the way his shadowy form appears in that grass for a moment. Somewhere where nobody would give him a second thought or even pay any attention to him, the way nobody presumably gave a second thought to the real Hwaseong killer. And I wonder if Joon-ho Bong did just that - or if, perhaps, he went the "Zodiac" route and used a different person in each scene where the killer appears, without having them show up in any other frames...