To find out what the real interests are in your negotiation you need to follow Roger Fisher's advice: separate the people from the substantive issues.
Over the years, I have been an observer and adviser to many global negotiations with individuals and companies that fail to heed this advice, and they always fail.
You can only negotiate with a decision maker; anything else is just influencing or persuading.
Focus on interests, not positions. People will often state a position, but you need to know what interests are driving their behaviour. 'I want at least a 15% discount on this new contract' is a position. You need to understand what interests are driving that position.
Internally, your best team member could approach you and state assertively, 'I want a 10% pay increase, or I am out of here!' Again a very firm position, but they have not shared their real interests. Never start negotiating from a stated position, as it will descend quickly into conflict.
The Program on Negotiation at Harvard has some clear advice on dealing with interests. At the negotiation table, the best way to uncover your negotiation counterpart's hidden interests is to ask questions and then listen carefully. Always open by asking and listening to assess interests, even if you've decided to make the first offer and are ready with some alternatives. Note that if your style of listening isn't sufficiently empathetic, it won't elicit honest responses.
You'll have to ask a lot of questions to get a clear picture of someone's interests. To model the type of response you're seeking; you must be willing to reveal your interests. Practitioners often assume that exposing their interests will give the other side an unfair advantage, but this is rarely true.
In live consulting, the question I recommend you use to uncover the interests, 'Help me understand...?' It is powerful because it is neutral in its tone and delivery.
If your efforts to uncover the other party's interests fail, try a new tack. Suppose that you're an account manager in a very large B2B negotiation with a key decision maker. You ask, 'Are you more concerned about the cost or the quality of our services?' Her reply, 'Both!' It will then start the conversation moving to uncover the real interests.