Akira Kurosawa, Shinobu Hashimoto, and Hideo Oguni
The three screenwriters for Seven Samurai were all staples of the Japanese movie industry, and the two not named "Akira Kurosawa" were his go-to guys for getting scripts together. Shinobu Hashimoto got his big break co-writing the screenplay for Kurosawa's Rashomon, the film that put him on the map and influences everything from The Usual Suspects to Diff'rent Strokes. Oguni did much the same thing, first teaming up with the master with 1952's Ikiru released a couple of years before Seven Samurai.
The trio got together for the first time on Seven Samurai, and their writing style was pretty basic. As Hashimoto tells it, he and Kurosawa would actually do the writing and Oguni would give them direction on whether they're stuff stunk on toast or not. He provided the structure while they filled in the details, which seems weird to Westerners since Kurosawa was ostensibly the big cheese in the room. Japanese culture stresses collective work, however, which means the boss man knew when to take advice on writing from someone he thought was better at it.
For Seven Samurai, they pulled out all the stops, locking themselves in a hotel room for six weeks and not stopping for anything but food and sleep. According to film critic Kenneth Turan, they even refused phone calls, which means that the studio, their wives or anybody else had to actually go bang on the hotel room door if they wanted to contact them (source). That was part of a longer three-month pre-production schedule, which no one in Japan had even considered before.
It was clearly an intense few weeks, but it paid off. Moreover, it established a work pattern that the trio would continue, along with the occasional extra man, for a number of Kurosawa's later films. Their ranks include Throne of Blood, The Hidden Fortress, and I Live in Fear… none of which are exactly chopped liver.
You can see this trend in a fair number of filmmakers, who use the same support team over and over again. (Watch a few Steven Spielberg or Martin Scorsese movies in a row, and you're apt to spot the same names on the opening credits.) In this case, it set the pace for the awesomeness to follow; with the screenplay, Kurosawa had a foundation on which he could tackle the rest of the film.
The screenplay itself is a fairly sparse affair as far as dialogue goes. These guys don't talk a whole lot, and when they do, it comes in short, to-the-point sentences. Most of the actual storytelling comes in the form of action, with the individual description of shots moving the narrative from point A to point B.
A lot of Hollywood blockbusters follow the same formula—we don't go much for talking in the Michael Bay flicks—but Seven Samurai goes into a whole lot more detail. It really is visual storytelling, instead of just showing us pretty images to cover up for the fact that we don't want to hear the actors when they open their mouths. The emotions of the characters, the pain they feel, and the greater forces they represent are all down there on the page, giving Kurosawa the perfect road-map to do his thing.