People will always act differently when they are with different people. We are social animals and we tend to adopt the attitudes and demeanor of the people we are around. It's called "mob mentality" but it has both good and bad sides. On the bad side we might do things we would never consider doing with others people on the good side this can mean broadening our life experiences in good ways. For instance: I have extreme social anxiety but when I am in a crowd of theater people I can express myself eloquently and freely...I get tongue tied in the supermarket talking to a cashier!

It may not be fake at all it is just another side of the person that you are seeing...if the actions are clearly contrary to the way the person represents themselves to you when you are alone I would back up a step and perhaps ask them about it in a non confrontational way. For example I saw a guy actually push a woman on her ass in a parking lot when he had attended a seminar on spousal abuse and swore he was non violent. I asked him casually when he believed it was appropriate to get violent and he indicated that he only got that way when he was drinking! SO ya dropped all contact with him and quick....he had told me he had been drinking the night I saw him get physical with the woman.

Anyhow be wary of forming opinions about people on this subject without giving them the chance to explain...but confronting them angrily will only result in defensive behavior. That shy person who has trouble making eye contact COULD be the star of the next community theater production...and still be telling the absolute truth!