Dien,
I once thought of writing a book on how to do interviews as you've described, because I've done a few ... but then I found there's a book already written. If you don't have it, you should get it, because it's really good.
It's "How To Create A Product In 3 Hours That Customers Want To Buy" by Don Bice, available at the link below.
Richard
"How To Create A Product In 3 Hours That Customers Want To Buy"