In my spare time, I love watching anime (Detective Conan, One Piece, etc) and Japanese drama (Liar Game, Hana Kimi, etc). After watching lots of Japanese shows, I started noticing how Japanese market their products. It is very different than how Western does it.
Their advertisements are embedded within the drama. So while the character does something boring or routine (such as driving or locking the apartment door), big letters would appear and names of sponsors would be displayed. Audiences will sit through all the sponsor names for sure, because the drama is still playing in the background.
They also market their actors differently. Some actors are dedicated to acting only (Oguri Shun, for example). Some do modeling on top of acting (Shibue Joji). Most, though, are also singers (Yamashita Tomohisa). If the actor is a male singer/actor (also referred to as idols), then chances are, he "belongs" to Johnny's.
I don't really know the details since I don't speak Japanese, but as far as I know, Johnny's Entertainment recruits boys to be idols. The boys start at a young age and they are taught dancing and singing. If they get better at performing (usually in their teens), then they will make a debut as a member of newly created boy's band. An actor can even belong to two or more groups at the same time. I don't know if it has rivals, but as far as I know, most (all?) boy's bands are managed by Johnny's. Hence, during the variety shows or performances, the idols can perform other groups' songs. No need to worry about copyright. And since they are under Johnny's, the different groups get along quite well. The seniors freely offer advices to their juniors.
Anyway, back to the singers/actors themselves. The first time they starts acting, the singers usually have very weak acting skills (in my opinion, their debut characters usually closely resemble their own personalities). But people still watch the drama. Partly because of the story. But mostly because they want to support their idols. And once they fall in love with the idol, they would search for other dramas the actors are cast for. The more popular the idol, it's more likely that their movies will be watched.
The fans know about their personalities and their lives. There are shows where they describe what they do in their spare time or for special occasion, how they interact with other groups, etc. But in spite of this, they still can go to regular school. I don't know how their life is at school, but apparently even the most famous idols attend regular university, take the regular train everywhere, and can take vacation without having to reserve the whole ocean for themselves (again, this is from what I know via watching TV etc).
I have this impression that Japanese like their idols to behave well. When an idol got caught underage-drinking, he would be reprimanded and penalized. He would not be allowed to go on stage or release an album. Although he had a lot of fans who wanted him to perform again, he could not escape the penalty of his actions.
In North America, scandals sell. Even with all the drugs, partying, and underage drinking, an actor/actress who is popular will never be punished. It's weird that people here seems to like the story of celebrity mess-ups.