Terribly difficult question.
If you don't know the answer that is.
PC Stout was completely flummoxed when a cyclist he had stopped for (allegedly) running a red light in London turned on him and asked on what basis he was being challenged.
Forget the fact that running red lights is in itself a dangerous act. You can even forget the fact that the policeman did not caution the cyclist (which apparently you have to do if a suspected criminal act has been observed). But what really did it was that PC Stout forgot the law that was contravened by said act.
When asked the pretty obvious question by the cyclist, which law have I broken, or on what basis are you stopping me, the officer could not accurately give a response.
All captured on the cyclists camera fixed to his helmet and broadcast on YouTube, the event has become both an embarrassment and reminder to us all to make sure if put on the spot we are able to explain or justify the basis of our actions and proposals.
For the negotiator you must expect the question of how you have arrived at your proposal. If you forget, haven't even got one, busk it on the spot, or be outrageously unrealistic expect the other side to ride off into the sunset.
Rather like the cyclist in the story.