I had a project recently where the director was lifelong friends with a break_out star, who had committed to doing the film. The agency was non_committal. Why? Because the agency didn't want to tie_up a month of the actor's time for a measly million bucks, when they could be getting 5┦x that from a studio for the same time slot. At the end of the day, a lot of money and time was spent pre_selling territories and shoring up the financing, only to have the star drop out, under target CAA's ('er I mean a random agency's) pressure.
There are enough litigation examples on this issue around to suggest their fears are well placed. A confidentiality agreement (also called a non_disclosure agreement) and followed by a letter of intent is the two step process for allowing a business concept owner to establish a new business relationship without losing control of their business idea.