The reason your boss acted like a human being during the interview is that if he acted like himself, neither you nor any other reasonable applicant would have accepted the job. He is obviously a pretty good actor, although he has trouble staying in character during the second act.
When dealing with a manager who adheres to the Jekyll and Hyde theory of management, there are a few steps to consider. In the first place, the next time he is mean to you, let him see how his behavior is working against his own objectives. He needs to understand that by being mean to you, not only is he generating stress and dissatisfaction, he is also undercutting performance and productivity.
One of the most powerful words in the persuasive process is "guarantee." Tell him that you guarantee that if he can treat you more professionally, your productivity will improve. Be sure to give him a clear idea of what you mean by professional treatment.
Over the long run, the best way to avoid this type of problem is to screen your future boss as carefully as he or she would screen you. Sometimes a few words with your future co-workers can tell you whether the boss knows how to act like a manager or merely knows how to act.
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