Phil suggests asking the candidate what she is passionate about, to see how excited she gets. Phil's idea is that you want to hire someone who can actually get excited about something.
Steve agrees that "simply finding out if someone can 'get passionate' about a topic is telling." He adds the step of asking the candidate, "How would you see that kind of excitement carrying over into your work."
Here's a third way to use a candidate's passion to discover a bit more about what makes her tick.