What do they do?

It's like asking, give us an example of your daily life. Obviously it varies from person to person, culture to culture. But what Japanese people do is different, strange to some and comforting to others.
The Japanese are reserved, usually keeping to themselves on the streets and on public transportation. It's rare to see people making short conversations or even smiling and saying hello to each other. It's nothing bad but probably out of manner. However, it isn't abnormal for the first train on the weekend to be packed full of people from clubbing the night before.

I've especially noticed a difference with Japanese in retail versus America. When walking into an American retail store, you hear the usual greeting and being asked if you need help. In Japanese retail, you always hear the welcome and leave you alone. In some places, usually high end stores, the sales people talk about how well their product is or how great it would look on you. With my lack of knowledge in the Japanese language, I tend to just smile and nod and occasionally say あそですか！ But their body language and way of saying things makes me interested in the product and I become more intrigued to buy it because I could go back home and have someone translate or help me with it's use if needed.
I've heard from Japanese students "Oh, so and so is like that because they are from Osaka, Osaka people are like that." Just as Americans have something like southern hospitality, people from different prefectures and regions have their own characteristics apart from everyone else. There are the characteristics that Japanese share among themselves and there are the characteristics that people in a town, city, or prefecture share as well.

Something that I had mentioned in a previous post is how friendly the Japanese are towards foreigners. Some Americans are indifferent to foreigners, many are frustrated when they cannot understand their english or cannot speak it at all. I also get frustrated because while we are in Japan, I notice that some foreigners also don't make the effort to speak Japanese, or at least communicate with some compassion. Perhaps it's just the culture of people to be confident within their own language without trying to make a fool of themselves by making the effort of speaking a different language other than their own.