Quoted from TheLaw:Playing a game next you isn't an issue.
And I'm going to disagree with this to an extent. If there's 12 machines, well then, fine: the adjacent machine is very likely to be played on. In a room/facility with darned near 100 machines and only enough people to occupy around 10% of them, it's kind of silly to go up to a machine next to a person that's playing and basically act the fool. If you know that you're, uh, "animated" when you play, you should at least try to hold off on playing until their first full game is done.
Even if you're not animated, a little personal space is never unappreciated, especially when there's lots of other machines you could be playing. It's kind of stupid to crowd things up "just because".