How to interview a faculty candidate?
If you have a 30-45 minute slot (which is typical for such things), you have to make some decisions:


Are you a 'buyer' or a seller' ? Are you evaluating the candidate, or also trying to convince them that your institution is a good place to work ? In general, you want to do both, but which you're thinking of emphasizing more will affect how you conduct the interview.
Is the meeting before or after the candidate's talk ? If after, you can do some probing based on the talk. If before, you might choose to ask them about material not in their talk, but in their CV. 
Is there concern about their area and how it fits with what you're looking for ? 
Have you heard anything from your colleagues during other interviews that might prompt you to take a particular line of questioning ? 


In general, since you can't really get a big read on a candidate in a short session, your best bet is to focus one one aspect (such as one of the above), and explore that in detail. One thing I've also found useful is to ask questions "out of left field" that test the assumptions the candidate's work rests on: if done right, this allows you to explore the candidate's larger views about their field and the direction of the research.