You should understand what type of difficult Customer they are and then to use the appropriate approach. With the right approach, even the most frustrating individual can be served with a minimal amount of stress and work on forming a relationship with them. You may want to ask conversational questions to put them at ease, and then move on to open-ended questions that will get them talking more. You’ll have to take proactive measures. For Know-it-alls, you should focus on delivering the facts. Don't give them a chance to one-up you up their knowledge.