My most important lesson in working with clients as a programmer, is that asking them how they want their program to work is like asking a native speaker to tell you the rules of grammar in their language. They have often lived with the rules of how their business process works for so long, it is intuitive, and no longer available to their conscious understanding, if it ever was.
Show them a scenario, though ("X, Y, and Z happen, so the program does A and B") and they can usually immediately tell you if that's the wrong or right thing for the program to do.
I have the impression that programmers, as a group, are mostly not great at understanding this, and it is a source of frustration for many of us when we program exactly what they told us to, and then it's "wrong". It is wrong. Like a linguist talking to a native speaker of another language, you often have to ask them about scenarios and piece together the actual business rules yourself.
The worst part is when the customer _thinks_ they know the rules, and then don't (consciously), and you have to help them through that cognitive dissonance of realizing that the business rules they "know" aren't actually the same as the ones they know.
