Observer K Huitula described the technical process: "They were painted with about 30 different colors of bottled poster color [a liquid form of gouache, also known as tempera]. On wet paper, first the basic color surfaces and tones are painted with a bigger brush, after which you move on to smaller details little by little."
"The straight lines are painted with a brush, taking support from a ruler and a stick gliding on its groove.
Here's an example of such a grooved-ruler device in action (thanks, Todd Bowlin)
"While not working on a precise project, all the free time is used for making painting exercises, taking advantage of the studio library consisting of various books on nature and photography. Also different variations are being done from the same painting, varying the colors and the impression of lighting."
Ghost in the Shell was based on a manga by Masamune Shirow. The film was produced by the company Production I.G., and the crew of about 200 people was directed by Mamoru Oshii. The backgrounds were the work of Hiromasa Ogura and his team.
The original film cost 5 million dollars to make. For the upcoming Hollywood live action remake, the fee for actress Scarlett Johannson alone is said to be 10 million.
Thanks, K Huitula!