What is good acting?

I’ve always liked Meryl Streep as an actress. I recently watched her in Julie and Julia. The first movie I remember her from was Kramer vs Kramer. It seems she can be cast into any role, flawlessly pull off any dialect and still come across as authentic and real. One commonality I began to notice in all the parts she plays is how she uses her eyes and her body. What she does is very subtle yet this is why it makes her so believable. An example would be at A Devil Wears Prada. It’s those cutting eyes looking over her glasses with her pursed lips “That’s all” https://www.youtube.com/watch?v=b2f2Kqt_KcE