First interview.....to find out the sort of person you are, be honest with your answers dont give answers you think the panel want to hear, and be prepared to state that you have no little secrets that could bring the magistracy into disrepute.
Second interview...you are given half an hour to look at 2 court cases, then be prepared to discuss what you would do with the offenders...the mitigating and aggravating features...once again be honest.
Good luck, it can be interesting, sometimes a bit scary and sometimes you will feel sad but its worthwhile.