Scott Bellware has a good post about how user stories/requirements need to be tested. Snippet:
Generally, we're taking too long to get stories validated - even in colloquial Agile development. Sending a user story through an entire development process in order to validate it is too much.”
Users can tell you what they want, but they can be wrong about what they tell you, for a multitude of reasons. This is one of the reasons why people do usability studies, as there is often a significant divergence between how users say they are using software and how they actually use it. Sometimes they literally forget about things they do to get around errors, e.g. “yeah, sometimes we get that error message but if we hit F5, it goes away".
Spikes, prototypes and simulations (of the iRise or balsamiq or SketchFlow) can be very helpful here (though you have to fight the tendency of prototypes being forced into production as ‘good enough’).
Some of his analysis seems to fall a little too much into the “there are two types of people, those that say that there are two types of people and those that don’t” category, but overall, it’s a good read. Check it out.