I have a reputation among my friends for being fidgety when watching a movie. Which is absolutely correct, and I tend to fidget because, well, I'm trying very hard not to talk. See, I love to talk during movies. To comment upon the action, or if there was something I particularly liked or disliked, or just riff on the movie MST3K style. I usually manage to control myself when I'm watching together with someone else, because my friends all hate me doing it. They just want to sit back and be absorbed in the movie, but I can't do that. I'm sitting there analyzing, making judgments and wisecracks as I watch.
I'm really trying to keep it internal, though, after one of my best friends got angry with me for ruining her movie experience when we watched Return of the King together -- it was during the scene when Sam rescues Frodo from Shelob, cuts open the web-cocoon and Frodo's staring, unhealthily pale face is exposed. That's when the words just escape me, without stopping by my brain first: "Oh no! It's Michael Jackson!"
This was, I must add, back when Michael Jackson was still alive. But my friend got really miffed, because that had been a very emotional scene for her, and she didn't appreciate the reference at that point.