Physical conditions must not be ignored. A hot stuffy room can negate the best material and the finest teaching techniques. Students who come in from a cold winter trek across campus may easily become drowsy in a warm classroom. This worsens after lunch. The size and shape of a classroom, the seating arrangement, the lighting, sound, color, temperature, and humidity all have some influence on the learner's attention level.
Guidance is most helpful early in a learning experience especially in the development of a skill.
Guidance can best serve to call attention to significant aspects of a problem and to suggest generalizations. Excessive guidance by a teacher is likely to result in learner apathy, conformity, defiance, scape-goating, or escape from the whole affair. The learner must play an active role. That makes listing references impossible, but you should be aware that the information is not from any of Dr. Clegg's work.