As a freelance writer for 15 years I've done a ton of interviews and whenever I do an interview I want to make sure I do it right. That means having the right preparation, the right tools and the right questions.
To prepare for an interview you need to know what you know and what you don't know about the interview subject. That means doing background research about the person, product, company or service. There are a number of useful research tools (newspaper & magazine archives, libraries, press releases, etc) that I use to help me to assemble a list of the known facts about your interview subject. This way I can ask the right questions and conduct an interview that benefits the subject's comfort level and the time you have available.
Whether it's a Q&A (with a short intro) or a full-fledged article, specify your subject, word count and deadline for an eloquent and effective piece than meets your needs.