This essay [1] includes Gian-Carlo Rota's recollections of Church as a teacher at Princeton. Rota writes, "It may be asked why anyone would bother to sit in a lecture which was the literal repetition of an available text. Such a question would betray an oversimplified view of what goes on in a classroom. What one really learns in class is what one does not know at the time one is learning. The person lecturing to us was logic incarnate. ..."
[1] https://www.princeton.edu/mudd/finding_aids/mathoral/pmcxrot...