But is this a bad thing? Perhaps if I teach from a heightened alertness instead of a calm comfort, I'll do a better job teaching. I'll be sharper, and the educational experience for the students will be better.
After all, lecture is much more comfortable than discussion. When lecturing, one can get into familiar speaking patterns and cover familiar material. In discussion, a facilitating teacher has to be more flexible, innovative, thinking quickly about student comments and trying to help a student-driven discussion move in positive directions. Because one can't plan out all features of a discussion, a teacher has to be on edge. This is true whether students are very responsive (you must be sharp in allowing all students to share ideas and sharp in helping bring those ideas together in a useful way), or whether students are not responsive at all (for then you must figure out way to get responses, or ways to usefully manage the time despite unresponsiveness). And for the classes I teach (composition and literature), discussion is much preferred to lecture.
Again, this alertness is a good thing. That's not to say it is bad teaching to get comfortable and familiar with your material and and the way you address that material. But when discussing literature, I think the class is better for the students because I'm not comfortable. When I'm edgy and alert, I'm giving students more chances to provide their own insights, and I'm sharper at finding new and creative insights based on the discussion.