I can't just sit. I have to sit in some strange, contorted position, else I feel uncomfortable. For example, on a couch, I typically sit sideways across the arms. I was just reading a book for an extended period of time on a couch, and while I started out sitting upright, I went through gradual phases and ended up swiveled around 180 degrees so that my feet were dangling over the back somewhere, and my head was next to one of the armrests.
Same thing with chairs - just "sitting" is so bland. I have to sit cross-legged or cross one leg over the other, or at least cross my feet.
The same thing happens with laying down. If I'm trying to go to sleep, I can't just lay on my back with my arms by my side and my legs straight out. I always feel somewhat dead when I do that.
You can learn a lot about someone by how they sit. There are the open leg sitters: those ones who always look like they're leaning forward, their arms resting on their somewhat spread apart legs, doing something (anything) intently. Then there are those who will put their arms on the seat back of anyone who's close by. That always somewhat unnerves me: did they sit by me on purpose so they could do that? What if I wanted to do that myself? I can't lean back now - that would result in contact between my neck and their arm - completely awkward.
There are, of course, those who don't sit. They just won't: "Oh, do you want to sit down?" - "No thanks, I'm fine", even though they're not going anywhere. It's as though they want to make sure they can manage a quick escape, should the situation require it.
Arms are troublesome appendages; they always get in the way when sitting, especially if there's nothing to be done with them. If there's no food, no coffee, no tea, no hand to hold, no video game to play...what do you do with them? They just awkwardly idle about - there should, instead, be some sort of way to retract your arms when they're not in use, or, conversely, some accepted social practice of ways to link your arms the person sitting next to you in these types of situations. Instead, people check their cell phones, play with their keys...anything to keep their arms busy.