You're not supposed to be trying to be someone you're not. If you have to put in that much effort during your day job then you won't be happy. You just need to

You're not supposed to be trying to be someone you're not. If you have to put in that much effort during your day job then you won't be happy. You just need to relax and be yourself, it not only makes everyone more comfortable but it lets you and the company start to evaluate whether or not you're good cultural fits. You'd be pissed off if they mis-represented the culture there during the interview process, and they have just as much right to be pissed if you act completely differently (in a negative way) when you start working.