Lecturing is a strange thing. I've been doing it for a while now, but I'm still learning a lot. I gave my last undergraduate lecture for the academic year today, so I've been reflecting on the process.
Despite the fact that you are -- ostensibly -- just standing at the front and talking (with the occasional bit of laser pointer waving), it involves a surprising degree of parallel processing. I don't use notes during lectures, but I do make sure that the text on the slides is detailed enough to prompt me with points that I might not remember on the spur of the moment (this also helps students on the handouts, or course). But typically, you're trying to lead the audience through the points you're making, linking ideas together to form a coherent story, sometimes between lectures or modules, and preparing them for a new idea, so there's a lot of off-the-cuff improvisation around the bare content. I had an experience today when -- as I was talking -- I saw an acronym that I couldn't immediately translate. As part of my brain was talking out loud about the first section, another part was fishing about for the expansion of the acronym. This time (it certainly doesn't always pan out so well) it came up trumps just as I had to talk about the acronym, and slipped the speaking bit of the brain the right words very smoothly. Score!
I can't speak for the students, but when this process works well, I find it extremely satisfying, almost like a mental/verbal equivalent of dancing. When it doesn't work, it can be excruciating, stumbling over every idea and garbling explanations. Either way, it's exhausting, so I'm glad to have a bit of a break before the next round.