Which armrest is yours in a movie theatre?
When I go to see a movie, I look for a seat with both armrests available. When I go on a date, I don't give a hoot about the armrest. Otherwise, I suppose it just depends on a person's level of trepidation.
In practice, I've discovered that the armrests belong to whoever has the more dominant personality. Because I have near-zero levels of dominant personality traits, sitting in a crowded theatre usually means I don't get an armrest unless the person sitting next to me is extremely nice.
However, kudos to your inquisitiveness; it may keep one's rights on the armrest. In theatres and auditoriums, corner and first-seat occupants are more likely to get both armrests, whereas middle occupants are less likely.
When you go to the movies with your friends, it's better to skip a seat so you each have your own armrests. It's also much more comfortable as long as the movie theatre isn't full or nearly full. I've embellished some of his response, but he basically told us to skip a seat, and my friend was correct.