I have a few products and every product has a few satisfied customers. It is great when customers are indeed telling good stuff about your product, but it is not easy to get it out of them.
So I created a questionnaire to help them.
Some of questions that are in it:
Is there any good resource for creating an interview of such kind?
Based on your response to Martin, looks like you could use some pointers about creating useful surveys.
Ultimately, you need to understand your target base and perform the due diligence necessary to create a successful survey. Asking open ended questions like "how did the need come about?" without any specific direction will likely yield poor results.
Here's a link to Survey Best Practices distributed by Survey Monkey (not a software guide). Also http://knowledge-base.supersurvey.com/survey-audience.htm has some insights. http://www.qualitydigest.com/oct96/surveys.html is dated, but can be helpful as well.