Here is what he says about editing:
“I learned a mountain of things about editing from Yama-san, but I think the most vital among them is the fact that when you are editing, you must have the intelligence to look at your own work objectively. The film that Yama-san had laboured painfully to shoot he would cut to pieces as if he were a total masochist. He’d always come into the editing room with a joyful look on his face and say things like, ‘Kurosawa, I thought it over last night, and we can cut that so-and-so scene’, or ‘Kurosawa, I thought it over last night and I want you to cut the 1st half of such-and-such a scene’, ‘We can cut’ ‘I want you to cut!’, ‘Cut!’ Yama-san in the editing room was a bona-fide mass murderer. I even thought on occasion if we were going to cut so much, why did we have to shoot it all in the 1st place? I, too, had laboured painfully to shoot the film, so it was hard for me to scrap my own work.Yama-san is Yamamoto Kajiro, a Japanese film director who is now mostly known as the mentor of Kurosawa.
But, no matter how much work the director, the assistant director, the cameraman or the lighting technicians put into a film, the audience never knows. What is necessary is to show them something that is complete and has no excess. When you are shooting, of course, you film only what you believe is necessary. But very often you realise only after having shot it that you didn’t need it after all. You don’t need what you don’t need. Yet human nature wants to place value on things in direct proportion to the amount of labour that went into making them. In film editing, this natural inclination is the most dangerous of all attitudes. The art of the cinema has been called an art of time, but time used to no purpose cannot be called anything but wasted time. Among all the teachings of Yama-san on film editing, this is the greatest lesson.”
Kurosawa goes on to talk about Uma (Horses), “which I had co-scripted and which Yama-san had put entirely in my hands for cutting.”
“There is one place in the story where a foal has been sold and the mare frantically searched for her baby. Completely crazed, she kicks down her stable door and tries to crawl under the paddock fence. I edited the sequence most diligently to show her expressions and actions in a dramatic way.That’s a very interesting point.
Bu when the edited scene was run through a projector, the feeling didn’t come through at all. The mother horse’s sorrow and panic somehow stayed flat behind the screen. Yama-san had sat with me and watched the film as I was editing any number of times, but he never said a word. If he didn’t say ‘That’s good’, I knew it meant it was no good. I was at an impasse, and in my despair I finally begged his advice. He said ‘Kurosawa, the sequence isn’t drama. It’s mono-no-aware.’ Mono-no-aware, ‘sadness at the fleeting nature of things’, like the sweet, nostalgic sorrow of watching the cherry blossoms fall—when I heard this ancient poetic term, I was suddenly struck by enlightenment as if waking from a dream. ‘I understand!’ I exclaimed and set about completely re-editing the scene.
I put together only the long shots. It became a series of glimpses of a tiny silhouette of the galloping mare, her mane and tail flying in the wind on a moonlit night. All that alone proved sufficient. Even without putting in any sound, it seemed to make you hear the pathetic whinnying of the mother horse and a mournful melody of woodwinds.”