The interview is primarily a personality test for the judge to see if he or she can get along with you for the next year. Sometimes they will have you talk to their current law clerk. It is important to remember that the law clerk started in September, so if you are interviewing close to September, keep in mind that the clerk probably hasn't been there for that long, but it is still important to make a good impression on them, because the judge can take their input into account to a greater or lesser degree.
It really is about how well you get along with the judge. They saw your application, they are picking from a pool of candidates with generally similar skills, what they really want in my experience is to find people with whom they can work for the next year. It can be stressful, but if you relax and be yourself you should be fine. Less