Steve Blank has already covered this in depth in his book 4 Steps to the Epiphany, as well as a video here: http://startupweekend.wistia.com/medias/tao3s8hf7l

My approach is based on his outline, with a twist.
1) I try and conduct the interviews at the customers office or place of work, at minimum on their devide (computer, mobile phone) so I can ask to see how they work.
2) I never try and sell them on a the solution, but work hard to truly understand if/where the problem is.
3) Always provide guidance to the conversation, but ask open ended questions.
4) Ask questions like "What do you do 3 minutes before, and after, you do that action (or use our product)?

Other tips would be
- Write down the words they use. Metaphors and taxonomies are VERY important to ensuring your product is approachable.
- Use 3 simple slides: Problem, Current Solutions, Proposed Solution

Hope that helps.

Hi!

Im Co Founder of BOMBAcamp (www.bombacamp.com) a lean startup accelerator for spanish speaking entrepreneurs. We habe accelerated through our different programs more than 2000 entrepreneurs in early stage to validate their idea.

The best to do a customer development interview is:
1. Introduce yourself, what problem have you seen and if this problem relates to him (your customer).
2. Ask the interviewed person to talk about him self, what does he do, family, work, friends, hobbies..etc
3. Ask open ended questions as tell me how is a typical day, tell me the last time you had the problem, how did you feel, what did you do to solve it, how often do you suffer this problem.
Try to look for stories of past behaviours and see the difference between what they do and what they say they do.
4. Ask for feelings and ways they solve or hack the solution today.
5. Make a conclusion and ask for referrals pf people they know have the same problem.
6. Ask for their contact info to reach them agaik to show them your first mvp.

Hope it helps. If you need more info we can have a call!