Fun fact: When I briefly worked in a funeral home, I learned that dead people just chilling on a table will sometimes look like they're breathing. I'm not talking about actual movement, like fluids settling in the body or anything like that. It's a sort of optical illusion. I guess the brain is used to seeing people breathing so it fills in the blanks. I experienced this illusion myself and so did others working there.

I've also been focusing on characters who aren't talking during a scene but are part of the scene. It gets extra awkward when there is a large group. Eg. 8 main cast members on screen, one guy talking for 5 minutes without interruption and all 7 other members just stare at them. Those scenes start to feel really artificial when you look at the other characters who just stare and listen. It gets even worse if one or most of those characters is portrayed as an asshole. I'm here thinking like no fking way at least one person doesn't cut into someone else's dialog IRL.

since UHD/4K you can sometimes see their pulse in their neck

My new favorite is looking for a pulse. Some movie I watched recently panned to the dead guy, clear pulse on neck, but checks pulse says no pulse he's dead then it cuts back to him with his jugular still pulsing. Pretty funny to see

Or sitting on the couch with a laptop and searching up wikis, Google, fan forums or social media to read about the movie you think you are watching but are actually ruining for yourself because you're so distracted that you can't actively read the laptop nor enjoy the movie at the same time ... two hours go by and you can't remember much about the movie or the little bits of information you read the whole time.

That and always watching for background actors making weird faces or just doing strange things.