Usually they just do masturbation or a school girl getting raped, unless they get famous in the industry by name. Then they usually star in hardcore fetish and S&M as well. The Asian audience may not seem interested in anything fucked up like S&M, but it is there, only under the surface.
AV Idols tend to have more cute faces than nice bodies, although Japanese men are obsessed with boobs(and don`t seem to like ass at all)
"Idol" does not really mean that they are an idol of the industry. Even if they brand new and no one knows who they are, they are still called an AV Idol.
The men in AV are not called Idols, they are just part of the staff.
Also, one major difference from a porn star is that AV Idols are much more likely to appear other places in the entertainment industry, like game shows and guest roles on series. It is not exactly common, but more than in the States.