What are the main differences between theatricalism and realism?
I'd say that the main difference would be that realism prefers to "ignore" the audience while theatricalism acknowledges it.
Realism is more of a naturalist approach. It mirrors reality and reader/audience accepting that what they are seeing is real. Realism tries to portray the subject honestly, with no exotic, supernatural, or artificial elements.
Theatricalism, on the other hand, is more like a performance. Here, you know it is not real because you are aware that you are here to watch actors/actresses perform. It is obviously acknowledged that what you are watching is a play and it is directly presented to the audience.