It depends on the situation. A customer who is being difficult due to a misunderstanding that's partly the employee's fault, would be dealt with in a calm manner in which I would allow him or her to get everything off of their chest, apologize for the situation and reassure them that we will do whatever we can to appease them. However, in a situation with a customer who is being difficult due to violating laws (theft), I would be stern, stand my ground and explain what will be tolerated and what will not, in a respectful manner.