it depends whom you are talking to - be smart enough to get alert if someone starts asking too many critical questions (especially about business model) ... also you can share only the outline of the idea (to get a response) rather than sharing the real meat. sometime you can also keep it very generic and simple, for example i'm planing to start a social/dating portal - where people get responses right away for their match - now by knowing this no one can figure it out how exactly you are going to do it and how it is going to be different from the rest of the competitor and what will be your business model/revenue stream etc.