Most of the startups that I have mentored have gotten to their customer discovery/value proposition phase where they are required to prove or disprove their value proposition and they freak right out because they want a list of interview questions.

I always tell them to ditch the questions. Approaching customer discovery with a list of questions will completely limit your responses by setting their thinking around a specific solution and then you miss understanding if your solution is even an answer to the problems they *really* have.

Instead, go back to what is prescribed in the customer validation approach and do a "day in the life" analysis. Don't assume you know what the problems are. Start with a hypothesis and then go to your customers and ask them to describe their work processes, or their buying habits, or whatever it is in the context of your product, but let them drive. Most times, they will quickly get to describing what's challenging about this process and the first things they will describe are the biggest pain points. Of course you want to be armed with some questions you can ask to drill down further, as described in some of the responses above, but listen, listen, listen. Guide them and let them talk. People will complain more than they will rave and with a skilled facilitator or business analyst, you will be able to maneouver the conversation to discover whether they actually care about the problem you think you are solving or whether you need to pivot your offering/value proposition to address what they are *really* struggling with.

Bottom line - ask them to describe their life in the context of your offering. Don't lead them to focus specifically on your solution right off the bat.