All they have to know is exactly what they expect. They can pay you to derive the detailed requirements for them and then they can sign off on them. However, it must be written down somewhere what, exactly you have to do in order to consider your work complete and to get full payment.

Over the long haul, it preserves the relationship you have with your clients, and it saves on your consumption of aspirin, whiskey, Maalox, and prune juice. It's better for everyone, as it lets them know what they're going to have when you're done, and what it will cost, and it lets you focus on doing the best possible job for them.

I've had people come to me with what they thought was a minor change and had to explain that it involved a 25x expenditure of manpower, a 100x increase in hardware cost, and a delay of 2 years in their development cycle that was supposed to take 5-6 months.

Back in the days when I was "working for the man" I had bosses drop things like that on me in passing in the hallway. Needless to say, they got a detailed memo right away, spelling out the impact on schedule, budget and on the other work that was supposed to be in progress. Sometimes, all they got was a resignation.

RE