2. Be friendly.
But at the same time know that the headhunter isn’t exactly your friend. If he or she is on anyone's side, it's that of the client they're doing a search for. This is a business relationship, pure and simple, ideally with something in it for both of you.
3. Be honest.
If the headhunter has you in mind as a prospect for a job, hear him or her out and then offer a candid opinion of whether you’re at all interested. If you’re not, the headhunter won’t be offended, but in fact will probably be grateful that you didn’t string the process along. If you’re intrigued, but uncomfortable having this sort of conversation at work, ask the headhunter if you can make an appointment to talk later, when you can speak more freely.