Ask HN: How open to be about your startup idea while doing customer interview

The first step in starting a startup is doing the customer interview, getting to know the customers thoughts. One thing that is concerning me and I am kind of insecure about as of now is "Hinting the idea to so many people at the same time." As from experience "what can be copied, will be copied" So I am slightly concerned about it and wanted to know your thoughts. Actually people whom I encountered 2-3 weeks ago, they wanted to speak my mind out and I later realized their ulterior motives(probably its too naive a question, but it happened and I am not sure how to tackle it) thankfully I did not speak stuff out there. What are your thoughts on it? How do I decide on how much open to be?