The reason this film trumps all other Shakespeare adaptions to cinema is simple: it doesn't care a whit for the original, cerebral, figurative and image-laden language. It carves all the meat off the bone of the plot, and uses music, atmosphere, stark melodramatic performances, and visual imagery instead.
Some of the sounds -- the wind howling up and down Mount Fuji, where the film was located, the whinnying of the battle-mad horses, the solid percussion of the actors' bare feet running in measured syncopated blocking, the whistle and impact of arrows -- take on the force of characters themselves.
Yamada's Lady Asaji is bone-chilling. Her face barely moves, her pale white makeup is passive, and she has eyebrows drawn on two level of her forehead. When she finally breaks down near the film's end, endlessly washing her spotless hands in a room whose walls are covered with an innocent victim's blood (the notorious traitor Fujimaki killed himself in the same room, now known as the Forbidden Room), wailing that she can't get clean, the effect is almost euphoric. Earlier, she exits through a panel in the dwelling's wall, vanishing into pitch black, then returning seconds later with a pitcher of drink for her husband.
Mifune's Washizu scowls even when he is laughing.
mist, colossal trees dripping with rain, rich black volcanic soil and bulky fortress architecture
Noh thousand-year-old theatrical tradition
The Noh stage must have on it three pine branches and a symbolic Shinto temple-arch. In the film, shots are carefully composed to include tangles of branches in the foreground, and the vast entrance gate of Washizu's fortress serves for the temple arch.
A Noh play features a "doer" (Shite) and a "companion" (Waku) who plays a subordinate role. Washizu and Asaji are the Shite and Waku respectively. Elements in the Noh include a battle-drama (we get one here) and a so-called "wig drama", in which a female character dominates the action. This is the central portion of the film, in the quiet of the fortress quarters, when Asaji ruthlessly manipulates her husband's ambition. Every Noh play has a ghost which appears to the Shite, and the spirit in the forest fulfils that function. Noh plays are never original works, in that (by a venerable convention) they are re-workings of ancient legends. Kurosawa follows tradition by quarrying his tale from Shakespeare's play.
There is no western term to describe the stylized striking of poses so important in Noh. Our word "dance" is a crude word which approximates to, but does not convey, the grace of the Japanese art-form. Asaji, alone with the blood-stain, gives us a glimpse of this delightful ritual.
Finally, Noh contains an aural richness almost totally absent from western tragedy - the complex rhythms of stamping and percussion which accompany the spoken word. In the film, the rhythmic patterns of horses' hooves on soil, and Washizu's bare feet on the boards of the banquet hall, are meant to reinforce the mood as they creep into our emotions by subliminal insistence.