This is one of the most frequently asked cross-cultural questions in film discourse and one of the least satisfactorily answered. 'They're both Asian cinemas' is not useful. 'Japanese cinema is slower and Korean cinema is more intense' is closer but still incomplete. Here is the fuller answer.
FORMAL DIFFERENCES - THE IMMEDIATELY VISIBLE
The most immediately visible formal difference is pace. Japanese cinema - particularly the art cinema tradition running from Ozu through Kore-eda and Hamaguchi - operates at a tempo calibrated to the ma aesthetic: the deliberate pace that treats the pause as meaningful, that refuses to cut before the scene has fully arrived. Korean cinema - even the formally patient films of Lee Chang-dong - operates with a different relationship to dramatic intensity: the scenes build to moments of emotional or physical rupture that Japanese cinema tends to avoid or deflect.
The difference is not that Japanese cinema lacks intensity. Tokyo Story is one of the most emotionally devastating films ever made. It is that the intensity in Japanese cinema tends to be contained - expressed through restraint, through what is not said, through the specific weight of the pause that carries what cannot be spoken. Korean cinema tends to externalise: the contained grief eventually breaks through the social surface in ways that Japanese films are typically constructed to prevent.
THE CULTURAL GRAMMAR DIFFERENCE
The Japanese concept of wa (harmony, group cohesion) shapes the specific formal quality of Japanese cinema's relationship to conflict. Direct confrontation between characters - the kind of scene that drives most Western and Korean drama - is structurally avoided in Japanese social culture and, correspondingly, in Japanese cinema. What Korean cinema would stage as a confrontation, Japanese cinema stages as a scene in which both characters demonstrate their awareness of the tension while observing the social norms that prevent its direct expression.
Korean cinema has han (the accumulated historical grief and resentment discussed elsewhere on this site) as its specific emotional baseline. Japanese cinema has mono no aware - the bittersweet awareness of impermanence, the specific sadness of beautiful things that don't last. These are different emotional starting points with different formal consequences. Han tends toward eruption; mono no aware tends toward acceptance. The formal registers of the cinema reflect the emotional registers of the cultural inheritance.
THE HISTORICAL CONTEXT
The historical relationship between Japan and Korea - including Japan's annexation of Korea from 1910 to 1945 and the specific cultural and linguistic suppressions of that period - is part of the context for the formal differences. Korean cinema's relationship to intensity, to the body, to social rupture reflects a different historical experience from Japanese cinema's relationship to restraint, to surface maintenance, to the social architecture that holds things together.
The Korean thriller tradition that emerged in the 1990s and 2000s - Bong, Park, Lee - emerged from a democratising society that had recently lifted decades of censorship and was working through its specific historical trauma in newly available formal terms. The Japanese cinema of the same period was working through a different trauma: the economic bubble's collapse, the specific way that the social architecture of the corporate-national consensus had failed. Different historical traumas, different formal responses.
THE GENRE RELATIONSHIP
Korean cinema uses genre - thriller, horror, comedy, melodrama - as a formal container for social and political argument in a way that Japanese art cinema often doesn't. The Bong Joon-ho film is always a genre film doing something else with the genre's conventions. The Hamaguchi film is often not a genre film at all.
Japanese genre cinema - the yakuza film, the samurai film, the J-horror tradition - is formally distinct from the Japanese art cinema tradition in ways that are more marked than in Korea, where the same directors move more fluidly between genre and art registers. The formal coherence of the Korean cinema tradition is partly the result of this genre porousness; the Japanese tradition is more formally stratified between the commercial and the artistic.
Han tends toward eruption; mono no aware tends toward acceptance.