I prefer to have a two-person team to do interviews - one person to conduct the
interview, the other to take in-depth notes and identify follow up questions.
This can be less critical if it's possible to record the interview, but I find
that the written notes are fantastic for sharing the takeaways with others
afterwards.
I found this post pretty thorough in terms of a process, questions, etc:
https://labs.openviewpartners.com/how-to-conduct-win-loss-analysis/#.W2OIs9hKjVq
I referenced it when writing about a broader win / loss analysis process:
https://www.crayon.co/blog/how-to-do-wi...