I have gone through the entire PSC process twice (once in Jan 2003 and again in Dec 2008) so here's what I recall about both times. It’s a couple hours long (!! Bring water) and you’re talking most of the time. Based on what I remember, here’s what they ask:
They ask you about your relationships with family, friends, significant other (if there is one).
Reflect on yourself: weaknesses, strengths, where you are in this life, what you’re looking towards
Reflect on significant incidents in your life that reveal who you are and your working style: a time when you had to resolve a conflict among team members? Or lead a team towards a difficult goal? How do you deal with subordinates who fail you? Have you ever had to side with one friend, against another? What did you learn from that incident, and is there anything you’d have done differently? Have you ever had to work with someone drastically different from you? Use your imagination to come up with other questions in a similar vein.
Reflect on significant periods in your life: what did you learn in college/JC? Significant friends you made and how they impacted your life? Most rewarding endeavors?
Basically, it would serve you well to sit and be very, very honest and introspective about your life, yourself, your motivations, and how you’d fit into this larger world. Write down your thoughts if it helps; if you consistently keep a journal/diary, now’s a good time to re-read it. Introverts tend to do better at this sort of reflection, but extroverts can easily pick it up as a good habit: not just for interviews, but for life in general. It always helps to pull back periodically and try to figure out where you’re headed.
There are also the PSC psychometric tests, but more on them
here.