Among the many challenges of teaching my Intro to Rome class is the fact that, well, I have to be the one to teach it. On the one hand, because I have recordings of all of the content archived, this isn't really a problem. If I am sick, I can post a recording and know that the students have at least been able to listen to me lecture on the content. In some real way, though, this misses the point of the class, which is to provide the students the opportunity to practice and apply the content. Delivery of content is a fairly small part of what I do in any class session. My PPTs are a mix of i>clicker, peer discussion, group discussion, and content slides. I ask the students to consider questions from previous lectures as well as from the content I have just delivered. I review main points of their Piazza discussions. It is not really possible to ask a colleague who has never taught this way to step in and take over for me.
Unfortunately, thanks to a dental emergency just before spring break, I had to do just that. Fortunately, I had already created the PPT presentation. I had a recording of me lecturing on the content slides. If possible, I wanted class to meet as usual; so I asked one of my teaching assistants to step in for me. This had several advantages, not least of which was the fact that he was used to the large audience and the method of teaching. He could take my PPT, listen to my lecture and take notes, and then try to approximate that in class in my absence.
Typically, graduate teaching assistants struggle with the lecture format. It's not an intuitive way to teach and requires very good time management skills as well as an ability to be clear, focused, but also entertaining. Too often, grad students spend too much time on early material and never get to the final third of their planned lecture. In this case, my TA delivered the lecture perfectly. He got through all of the material; he was lively but got all the key content across to the students; and he injected his own observations from time to time. It struck me that this mode of teacher training is far more effective than our more usual "here's a topic; create a lecture and deliver it" mode. In this instance, the TA could see the things that I thought were important to emphasize but he could also add his own twist. It was ultimately his lecture, not mine; yet it covered all the necessary content and kept the class on track.
Many professors I know are taking MOOCs for much the same reason: we want to see how someone else teaches the same material we teach. This is going to be a valuable contribution of the MOOC. Too often, once we get jobs, we get little useful feedback on our teaching and rarely have conversations with others who teach our same courses (typically because we are the only person at our institution to teach that course). The internet has changed this to an extent, in that we can now look at syllabuses from colleagues' courses; but being able to audit them adds an entirely new dimension. In the same way that (I think) my TA was able to learn from "watching" me and then doing it himself, I have learned a lot from watching my colleagues at other institutions teach courses on topics like Greek mythology.