The following quotation is from George Stigler's collection of essays on academia and society, "The Intellectual and the Marketplace".
The good teacher is a mysterious person, and yet we must know his character before we can prescribe his training. In my view, the good teacher is not distinguished by the breadth of his knowledge, by the lucidity of his exposition, or by the immediate reactions of his students. His fundamental task is not to dispense information, for in this role he is incomparably inferior to the written word. His task is to fan the spark of genuine intellectual curiosity and to instill the conscience of a scholar--to communicate the enormous adventure and the knightly conduct in the quest for knowledgeIn other words, research and teaching are linked. One wonders what powers that be in our universities would make of this.
To this end, the fundamental requirements of the good teacher are competence (How can the incompetent be other than slovenly?) and intellectual vitality (How can the sedentary excite us to bold adventure?).
These traits may be acquired by wide reading and deep reflection, without engaging in research and becoming a specialist. But it is an improbable event. It is improbable psychologically: it asks a man to have the energy to read widely and the intellectual power to think freshly, and yet to do no research. He is to acquire knowledge and construct ideas--and keep them a secret. It is improbable scientifically: it asks a man to be competent in his understanding of work that he has had no part in constructing. At lease in economics, this is almost impossible. There is no book that states the consensus of the profession on the ideas that are changing--and these are naturally the most interesting ideas. Only the man who has tried to improve the ideas will know their strengths and weaknesses. Scholarship is not a spectator sport.
(HT: Division of Labour)