Here is an apt description of what lecturing to a large professional school class is like, in this case to medical students. Certainly, there is no better description in all of literature and this one comes from Anton Chekhov's early famous story "A Boring Story," written when Chekhov was 29. His plays and other great stories were yet to come before his utimely death 15 years later.
"I know what I am going to lecture about, but I don't know how I am going to lecture, where I am going to begin or with what I am going to end. I haven't a single sentence ready in my head. But I have only to look round the lecture-hall (it is built in the form of an amphitheatre) and utter the stereotyped phrase, 'Last lecture we stopped at . . .' when sentences spring up from my soul in a long string, and I am carried away by my own eloquence. I speak with irresistible rapidity and passion, and it seems as though there were no force which could check the flow of my words. To lecture well -- that is, with profit to the listeners and without boring them -- one must have, besides talent, experience and a special knack; one must possess a clear conception of one's own powers, of the audience to which one is lecturing, and of the subject of one's lecture. Moreover, one must be a man who knows what he is doing; one must keep a sharp lookout, and not for one second lose sight of what lies before one.
"A good conductor, interpreting the thought of the composer, does twenty things at once: reads the score, waves his baton, watches the singer, makes a motion sideways, first to the drum then to the wind-instruments, and so on. I do just the same when I lecture. Before me a hundred and fifty faces, all unlike one another; three hundred eyes all looking straight into my face. My object is to dominate this many-headed monster. If every moment as I lecture I have a clear vision of the degree of its attention and its power of comprehension, it is in my power. The other foe I have to overcome is in myself. It is the infinite variety of forms, phenomena, laws, and the multitude of ideas of my own and other people's conditioned by them. Every moment I must have the skill to snatch out of that vast mass of material what is most important and necessary, and, as rapidly as my words flow, clothe my thought in a form in which it can be grasped by the monster's intelligence, and may arouse its attention, and at the same time one must keep a sharp lookout that one's thoughts are conveyed, not just as they come, but in a certain order, essential for the correct composition of the picture I wish to sketch. Further, I endeavour to make my diction literary, my definitions brief and precise, my wording, as far as possible, simple and eloquent. Every minute I have to pull myself up and remember that I have only an hour and forty minutes at my disposal. In short, one has one's work cut out. At one and the same minute one has to play the part of savant and teacher and orator, and it's a bad thing if the orator gets the upper hand of the savant or of the teacher in one, or vice versa.
"You lecture for a quarter of an hour, for half an hour, when you notice that the students are beginning to look at the ceiling, at [the teaching assistant]; one is feeling for his handkerchief, another shifts in his seat, another smiles at his thoughts. . . . That means that their attention is flagging. Something must be done. Taking advantage of the first opportunity, I make some pun. A broad grin comes on to a hundred and fifty faces, the eyes shine brightly, the sound of the sea is audible for a brief moment. . . . I laugh too. Their attention is refreshed, and I can go on. No kind of sport, no kind of game or diversion, has ever given me such enjoyment as lecturing. Only at lectures have I been able to abandon myself entirely to passion, and have understood that inspiration is not an invention of the poets, but exists in real life, and I imagine Hercules after the most piquant of his exploits felt just such voluptuous exhaustion as I experience after every lecture."