I know celebrities with have done AMAs, and I can say with the exception of proxy PR people coaching (usually when they have a movie coming out soon or something), you end up getting pretty honest and direct responses on reddit from them.
All of them, because it is ridiculous to assume that you know what every person in a particular group of people is like in real life based on the actions of a subset of that group on a message board.
Some celebrities "get" reddit, and some don't.
Some answer three questions and leave, and some stay for days.
Some see it as a promotional tool, and some are actually part of the community.
Some participate anonymously, some have AMA accounts, and some have both.
Most of all, even if you did know a good portion of them on and offline, you can't possibly extrapolate what the rest of the group is actually like in real life.
The idea that you can encapsulate an entire person into a couple dozen comments (on average) that they leave on a message board is absurd to me.
Going on to extend that abstraction to an entire class of people is even more ridiculous.