Once you see that an answer is not serving its question properly anymore, it should be tossed away. It's just their natural life cycle.
They usually kick and scream, raising one hell of a ruckus when we ask them to leave. Especially when they have been with us for a long time.
You see, too many actions have been based on those answers. Too much work and energy invested on them. They feel so important, so full of themselves. They will answer to no one. Not even to their initial question!
The hardest thing about customer interviews is knowing where to dig. An effective interview is more like a friendly interrogation. We don’t want to learn what customers think about the product, or what they like or dislike — we want to know what happened and how they chose... To get those answers we can’t just ask surface questions, we have to keep digging back behind the answers to find out what really happened.
In this sense, trust is a polarizing strategy, and it's one that is important to apply early on in the relationship before someone becomes important to you. If you trust someone excessively and it goes badly, but they don't matter to you, you can just kick them to the curb. In general, trusting someone at a level that seems slightly excessive for their level of importance to you will help you sort people in your life who you want to be more important to you than they are from those who you want to be less important than they are.
And it does need to be excessive. It needs to be trust beyond reason. Not beyond all reason, but somewhat beyond what currently seems reasonable. If it is not, then unless they are prepared to take the first move, you will never find the signs you need to move to a higher level of mutual trust.
Sometimes this will go badly, but you need to be able to try bad things.