The preparation for class began long before, perhaps months. This may also keep being adjusted as new and relevant information come to light. The instructor needs to be prepared for the class at all times. Sometimes the instructor may have to change the instructional method to adapt to a particular student way of comprehension, but at the same time not loosing focus on how to acheive the stated objectives.
Instructor should have command of the subject matter and be able to draw references and give examples. One of the biggest responsibility of the instructor is make sure that the students have the materials to do the work. So he/she has to be cognizant of what is consumed and note what is needed. The instructor has to be a manager for different aspects of the educational system.
