In the film, the octopus-munching scene takes place shortly after Dae-su is freed from captivity. He enters a sushi restaurant and tells the chef, Mi-do (Kang Hye-jung), that he wants to eat a living thing. She goes into the kitchen while he talks on the phone to his anonymous tormentor; Mi-do then comes back a few moments later with a live octopus on a plate. She offers to cut it up for Dae-su, but he grabs it and savagely tears into its mantle. He shoves the tentacles into his mouth, and they hang out like living spaghetti, grabbing at his face in a futile attempt to escape. It’s a primal scene, and shows how consumed with hate and revenge Dae-su is. He has made himself into an instinctual killer.
Watching the behind-the-scenes clip, the scene doesn’t look like it was fun to film. The actors and crew members seem to know that what they’re doing is pretty extreme, and they take it seriously. At the same time, though, there’s a little bit of dark humor at play, like they have to laugh about it in order to get through it. One take gets blown because Choi starts laughing while he’s chewing, apparently because he can’t bite through it, and he’s gnawing away at it like a madman. He and Park both collapse in helpless laughter. The magic of filmmaking, right?