Many employees don't know how to interview and you should teach them the basics as well as educate them on what you are looking to learn from their interview
If you're in the position of doing interviews for the first time, and don't have anybody around to teach you the basics, I would thoroughly recommend getting a copy of Lou Adler's book "Hire With Your Head". It has some really excellent advice on how to approach interviewing, and the hiring process in general.
There's a brief article on Adler's site that covers accomplishment based interviewing that you can find at http://www.adlerconcepts.com/resources/column/interviewing/u....
Having a focus on accomplishment based questions - drilling down into a candidates' experiences on real projects - is a hugely useful technique. Talking about the work in detail helps you separate candidates that interview well from those with real skills.