I mostly observe and watch things in movies that is not the focus of the movie:
I like looking around the set, seeing what products and items the characters have. What their houses look like. I am fascinated by sets that have a ton of books. In “The Shining”, Jack and Wendy’s apartment has a TON of books just stacked everywhere. And did you know that Danny has a goldfish? You can see it in his room in Boulder and for brief moments in their little apartment at The Overlook. But Kubrick is pretty visual, so it’s always a treat. For such a horrible person, Alex sure keeps a neat room in “A Clockwork Orange”. He rapes and beats people, but his mother never has to get after him about his room.
I always like to observe how people are talking on phones, and whether they are talking on it the “right” way (so the mouth is towards the audience). This is because when I first took an Acting class (and my only Acting class in high school), one of the first things she talked about was placement and she used the telephone as an example.
I like to see what the actors are doing with their hands or the placement of their hands. Mostly they’re just twiddling their fingers.