This is sort of a familiar one.
A client wants someone to build a website, and doesn't want to give them all the pieces of the puzzle at once (or is very vague about it).
There are three choices here:
- Continue being trickled information, eventually going insane.
- Leave that client instantly, without hesitation.
- Tell your client, plain and simple, that they absolutely have to give you all of the information you need or they will not have their site finished (without huge delays and vastly more money).
There's also a fourth, oft-cited but never usually done by clients that act this way:
Sign a contract, stating that you will not do so (aka a Non-Disclosure Agreement).
It's a really simple process for the client, but too often I see clients with no real business sense trying to protect an idea without a clue what they can do except hog the puzzle pieces and not give you them until you've solved the rest of the puzzle (which is a grossly unacceptable way to create a website, to be frank).
Explain to your client, in relatively simple terms, that keeping ideas and designs to themselves is the absolute wrong way to go about designing a successful website. Explain that many things on the website (skeleton layout, templating, major parts of CSS) cannot start without the entire picture in place. If they did start without the entire design, for example, you would be constantly remaking the site's skeleton layout from scratch.