one of my favorite moments in a stage performance that plays with the diegetic vs non diegetic is when Doctor Bartolo sings his arietta in Act 2 of The Barber of Seville to try to show the younger characters that music was much better back in his day. In most productions he sings it somewhere between amateurishly and badly. There is no spoken dialogue in Barber so when Bartolo isn't singing, his singing is excellent, and when he is singing his singing sucks