You need to understand that as the Project Manager, you have many customers, you have the external customers, and the internal customers. The experienced Project Manager learns to understand the customers point of view and then take that into account with his actions.
If you do not know what it is your customer really wants then it is possible that you will deliver something that he doesn’t want. You could even put 80% of effort into achieving something on the project that is only 20% important to the customer.
The challenges come when dealing with internal and external customers, it is highly likely that the two customers will have completely different priorities. This is where you must use your skill as both Project Manager and negotiator, if you understand what drives both parties then you will be able to establish the right course of action to recommend supported by factual information gathered.
Depending on your position in the company will depend on you next course of action, if the external customer is insisting on a certain course of action and this is at odds to your internal customer then it will be time to escalate to the correct authority internally within your organisation. This will normally be your boss or functional manager, despite your feelings on the situation, the call will be with him/her, but will probably be down to you to take the good news back to your external customer.