I strongly disagree with some of the advice you have gotten here. Some have said that the idea is not important and that execution is far more important, so don't bother with an NDA. I agree with the former part of that but strongly disagree with the conclusion. If you're approaching a subject matter expert in the field about your idea, they are far more likely to run on their own with this, than someone in the general public. I've actually seen it happen many times, and when it does the entrepreneur has absolutely no recourse. So my advice to you, would be to get the subject matter expert to sign an NDA. And if they won't as they want to leave their options open, then find another expert.