When interviewing an experienced candidate, you want to talk about her previous job. In this question, I'm looking for one thing: passion. When you find a project that the person worked on recently, here are the good signs:
She gets very excited talking about it; she talks more quickly and gets animated. This shows that when she is interested in something, she will be passionate about it. There are far too many people around who can work on something and not really care one way or the other.
Even if they are passionately negative, this can be just as good a sign. "I was working on installing Foo Bar Mark II for my previous employer, but he was such a dope!" These are good candidates we want to hire. Bad candidates just don't care and will not get enthusiastic at all during the interview.
A really good sign that a candidate is passionate about something is that when she is talking about it, she will forget for a moment that she is in an interview. Sometimes a candidate comes in who is very nervous about being in an interview situation -- this is normal so I always overlook that. But then when you get her talking about Computational Monochromatic Art she will get extremely excited and lose all signs of nervousness. Good.
Joel On Interviewing 7: Does She Have Passion?