One of the most important exchanges in Parasite could easily be missed on first viewing. The Kim family are bundled around a table having dinner, discussing their wealthy new employers, the Parks. Even though shes rich, shes nice, says Song Kang-hos Ki-taek between mouthfuls of noodles. Shes nice because shes rich, shoots back his wife. Soon, their meal is interrupted. As the film evolves into violent farce, that line is left behind but its sentiment hangs in the air.
That may sound like wordplay, but its a crucial line, explains Song, speaking through a translator at Londons Korean Cultural Institute. When youre rich, its much easier to be generous, and that translates to being nice. When youre less affluent, even if you want to be generous, its very difficult. 
Then again, nobody is really nice in Parasite. In Bong Joon-hos exquisite black comedy thriller a frontrunner in the Oscars Best Picture race the impoverished Kim family finagle their way into the lives (and home) of the much more affluent Parks, with increasingly brutal consequences. The cruel complications of class and wealth abound in a thrilling genre movie with a deliberately murky morality. 
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We first meet the Kims as they are folding pizza boxes for petty cash in their basement flat, son Ki-woo (Choi Woo-shik) roaming the tiny rooms as he tries to poach a neighbours wifi. But when his friend, about to go abroad to study, suggests that Ki-woo takes over his job as an English tutor for the well-to-do Parks by faking the right qualifications, the family spots an opportunity. Through some pretty inventive scheming, Ki-woo becomes the tutor, his sister Ki-jung the art therapist, his mother the housekeeper, and his father the chauffeur. 
They are moving up in the world. Literally. The Kims own flat is half-overground, half-underground, a sunken place frequently flooded with clouds of fumigation gas (one of several sideways glances to the parasite of the title). The Parks house, an architectural marvel built specifically for the film, is resplendently above ground, its upstairs the most coveted space of all. Its amazing symbolism, says Song, because the basement flat expresses the hope but at the same time the despair. Youre half overground, so theres a potential to move up, but theres also the potential to go to the absolute basement. Theres this irony that the film is dealing with, and I find it very powerful. 
The purpose-built Park family home in Parasite (CJ Entertainment)
Songs performance is powerful, too. A huge star in his native South Korea, having starred in several other Bong films including The Host (2006) and Snowpiercer (2013), the 53-year-old is known for playing a kind of pushed-to-the-brink everyman. Hes not dressed like one today hes clad in a cyan mohair jacket and a patterned silk scarf but that strange mix of otherness and familiarity is there. Songs face, with round cheeks that contradict the rest of his sharp features, has a defeated, hangdog air about it when at rest. But it can change in a flash to delighted; cheeky; wiley. At one crucial point in Parasite, it is shot through with undiluted rage though mercifully I dont see this today.
Though Parasite has made history as the first Korean movie nominated for Best Picture at the Oscars, none of its actors got a look-in. The New York Times Magazines E Alex Jung suggested that theres an old prejudice at work here that sees Asian people as technical workers hence the praise for Bong Joon-ho and refuses to see us as fully human. When we meet, the nominations are yet to be announced, but Song demurs when I bring up the subject. Thats Bong Joon-hos thing, he says, but its very honourable to receive certain awards. 
The Parasite cast accepting the SAG Award for Outstanding Performance by a Cast in a Motion Picture (Getty)
Back to the film, then. One of the films key relationships is between Songs Ki-taek and his boss, Dong-ik (Lee Sun-kyun). They have a cordial, even affable, dynamic on the surface. But it is ultimately transactional, and when Dong-ik thinks his driver cant hear him, he talks about him as though he is subhuman, cruelly but casually denouncing his smell. It is like an old radish, he tells his wife. No. You know when you boil a rag? It smells like that. Its a comment that will later trigger one of the films most violent confrontations. 
The smell is something invisible, says Song. Thats where the horror lies in this film. Because theres something very powerful, but you cant see it, but it determines the relationship. Thats where the horror lies, and thats where the power of this lies. 
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Still, he insists there are no villains in Parasite. I dont think Park is a bad person, he says. Hes relatively thoughtful, kind and considerate… but behind all this facade, between them theres a sense of hurt, and his pride constantly gets threatened and challenged, and this accumulates over time. 
So who is the parasite here? I think in some sense, says Song, every character is a parasite. The rich family rely on the driver, the housemaid and other people, so in a way, they are parasites as well. So no particular character is a parasite. I think everyone is. This film is about how we can live together, and how people can build symbiotic relationships, rather than pointing at someone and calling them a parasite.
To Song, there is no better summation of this than the statement Bong released when Parasite first came out. This film is a comedy without clowns, a tragedy without villains, all leading to a violent tangle and a headlong plunge down the stairs, he wrote. Who can point their finger at a struggling family, locked in a fight for survival, and call them parasites? 
Parasite is out in UK cinemas on Friday