I've been at two on-site interviews at Google, and that was the most exciting experience, especially the first time. Now I know how proper technical interviews should be done. In general, I realized how much more I'm able to do, compared to what I was doing at my place of work.
The general approach is to figure out how the candidate is able to think in a structured and analytical way. It doesn't matter if the candidate doesn't know something -- much more important is to see how he or she is developing the answers during the interview. Of course, the questions should also allow such freedom.
Also useful, is not to ask for definitions of something, but for difference: how A differs from B and why? Then you clearly see if the candidate understands what's behind it, even if they have forgotten some little details.