Developing PCK requires a certain level of subject matter knowledge, and teachers have a different understanding of subject matter than a person who specializes in that same field. A chemistry teacher and a ‘practicing’ chemist both have subject matter knowledge in chemistry; however, the knowledge is applied differently.
Josh Kenney's blog
Many researchers have attempted to characterize the knowledge of an expert teacher, and it's complex, to say the least.
I think that most people can recall someone whom we considered to be a great teacher. The kind of person who inspired us and motivated us to learn. As I started my career, I remember wondering what kind of teacher my students thought I was.