During the past couple of years I have observed teachers delivering a variety of lessons. Surprisingly, perhaps, most of these lessons have been in subjects very different to Physics.
The idea of a non-specialist teacher observing a lesson and, furthermore, providing feedback may seem a little strange if not, even, of little benefit. My experiences, however, over the past couple of years have transformed my own opinion of this.
Let's face it, it doesn't take an expert to recognise an outstanding teacher - or, for that matter, a failing one. But can a non-specialist actually provide meaningful feedback in a subject very different to their own? Well I would argue that not only can they do so, but they might be able to offer better quality feedback than a colleague from the same department. The problem with colleagues is that they often come along with their own preconceptions about how certain topics should be taught and their own expertise in a subject can, sometimes, also blind them to the problems that the pupils may face during lessons. A non-specialist, on the other hand, may be better able to empathise with the experience of pupils and, coming without the preconceptions of the experts, may be able to make novel suggestions that wouldn't occur to other teachers.
This approach has worked very well for me over the past few years - so much so that nowadays I readily welcome non-physicists into my own lessons and, furthermore, often find their advice and feedback more helpful than that of my closest colleagues.