When Does an Interview Turn into Consulting?

I had an "interview" with a company who apparently thinks they have come up with something interesting that is in my field of expertise. Five minutes into the conversation it was apparent the company had only a high-level understanding of the market, as I was running circles around them on what was going on in this particular segment. It is a market I know cold and have been involved with in one way or another for over six years.

The interview soon turned into Q&A, not on my abilities as a worker, but on the market, players, customers, competitors, etc. They are inviting me in for a follow-up face-to-face, and have forwarded me a Non-Disclosure Agreement (NDA) so they can talk more freely about their invention, so it will be interesting to see if the meeting turns into more brain-picking or something that moves forward into a full-time position.