Along with the good advice others have mentioned, let me add one thing. I find it useful (and actually enjoyed interviews where this was done with me) to bring a practical problem to the interviewee. For example, 'If you had to solve problem X, how would you do it?'. Then, give them a piece of paper to allow them to write (maybe a short flow chart, or pseudo-code). Then you can interact with them as you see their though process. This was done at an interview with me, and I had a good time interacting with the fellow interviewing. Not only could he see how I think in real-time, but I was also able to follow his thought/design process. It allows you to ask other follow-up questions like 'Why are you using module Z and not module A?', and 'But what if X occurs?', etc...

Cheers,
KM