For the type of talent/work you seem to have in mind, the deal is usually straightforward -- you book her, she shows up and does the job. The profit sharing arrangement complicates things a little because once you identify and select a likely spokesperson, you'll have to "sell" her manager on the idea before you can move forward.

How the manager views the deal relative to his or her own financial goals will be a more of a factor in whether you get a "yes" or "no" than what the celebrity thinks. So be sure to have very good financial projections ready -- the manager is effectively your "investor," and he/she will most likely be a money management and investment portfolio professional. If you come in with an amateur presentation or questionable figures, you'll get a flat "no" -- and not many second chances (since managers and booking agents all talk to each other).

Of course, fortune could work in your favor -- the perfect talent may be having trouble finding work (especially if the writer's strike continues), and her manager may have steady revenue coming in from other celebrities -- and therefore be open to having this client work with you under a more speculative arrangement. Sounds like you have a lot of fun ahead of you -- best of luck!