Authenticity

Authenticity is one of the most important aspects of teaching.  It is the gateway to honesty and fairness, and provides the basis for a good classroom environment.  In my opinion there are no limits to expressing authenticity.  To be authentic is simply to be what you say you are.  If you say you are a fair person and you intend to treat everyone fairly and you do so, then you are authentic.  Authenticity should not be confused with being an open book.  Being authentic does not mean that you must share every real thought and feeling you have every time you think or feel it; those things require censorship.  To be authentic you only need to be true to yourself, unless your true personality is mean, judgmental and generally negative.  In that case, being fake is the best shot you have at teaching anyone anything. So for argument's sake, let's assume you're a nice person.
A lack or loss of authenticity in the classroom can have an incredibly detrimental effect on students and their learning.  A student can always tell when a teacher is being "fake"; its instinctual. Once a student has detected that a teacher isn't being authentic, that student will most likely lose respect for the teacher. When this happens, it becomes more difficult for the teacher to do his/her job and the students will learn less than they did before and the classroom environment becomes more negative.
In general, if you feel the need to put on airs in order to be successful at your job, maybe you're in the wrong field.  Some people are better suited to office work than to teaching or better suited to a laboratory than an auto shop.  Whatever the employment we should always be true to ourselves, that way we have a shot at being happy. If you hate your job on a good day, you're doing the wrong thing.