If you're going to interview someone for a position, shouldn't you be knowledgeable about the technologies you will be covering in the interview? Maybe you can get a colleague to sit in with you on those parts and have him ask the WS questions.
In that case you're better off learning about WS rather than just memorizing answers to questions that may never come. For stuff you don't know it's perfectly fine to say so. (I'm assuming, of course, that you already know about WS and aren't trying to fake that knowledge.)