It’s not really uncommon for clients to know what they want to get from you, but at the same time have a very vague idea about what they actually need you to do for them. Then, of course, there are clients that only know they need a [specialist of your kind] but even their actual goal for this project is not well defined.
A question that I get a lot is- since I price on the work I actually do, how do I price these kinds of clients?
A quick answer: develop a few productized services, then make them choose one of them.
If that doesn’t work because they are sure they need something truly custom, consider another approach: Ask them questions to flesh out the project scope, but make sure they realize “I don’t know” is the most expensive and least convenient answer they can give.
People usually focus a lot faster if they know being unfocused could cost them more money. This is always true, but that’s not always clear to clients.
This will scare some people, of course, but probably not any clients you would be sorry to lose. Namely, the worst category of “custom” clients – cheapskates who are also vague about what they want – as they are likely to call you “unprofessional” and take their business elsewhere.
Isn’t that wonderful? Smarter people than me have said that marketing should be like a magnet – attract the right kind of clients, but also repel the wrong kind.