Use a fixed seven-question guide so answers stay comparable: how the business has changed in two years; what triggered the original search for a supplier; what they evaluated and what nearly stopped them choosing you; what a good and a bad week with a supplier looks like; where you make their job harder; who else would be on the list if they were choosing today; and what would have to change for them to give you more business. Follow every answer with a request for a specific example, which is where the usable detail lives.