I only did that one interview thing with my friends who had already applied. They were the ones that actually gathered the questions. It took me thirty minutes, and that was it.
Personally, there's nothing really to prepare for. Show up with a purpose and a passion with the confidence that you deserve to be at the interview while being yourself and you're already better than 95% of other interviewees. Be truthful to your morals and values. Other than that, the only real preparation you need is to just think about how you would answer hypothetical questions. I did this with a 30 minute mock with my friends, but you could simply do it by yourself in the shower / mirror / etc.