To get the most out of a lecture, it takes both an engaging speaker and an active listener. Yes, you as a listener are an essential part of the formula. It is not only the job of the lecturer to grab your attention and teach you, it is also your responsibility to actively listen and interact with the speaker to maximize your learning experience.
If you walk into an empty room with no idea of why you are there, you might be surprised and learn something. However, if you go into a lecture seeking answers to questions you have formed, searching for organization and meaning to material you have previously read and reviewed, you will learn on a whole different level.