Two occassions. First was when me and my mates went to see The Cave and were the only people in the theatre. The movie sucked, but we all sat and trash talked the movie and threw popcorn at each other (It was the salty popcorn that my mate bought, then decided he didn't like, so our violent popcorn showdown was not a waste).
The second occassion was when me and my mates, who are all Bond fans or action movie fans, went to see Casino Royale on the night in opened. The theatre was completely full. However, there were no chavs sat in the corner texting on their phones and being way too loud (such as during my second viewing of Avatar, not cool, but one guy eventually just stood up, in the middle of the packed theatre and told them all to "Shut the fuck up!", I felt like saluting him), the theatre was full of movie lovers or Bond fans. The atmosphere was brilliant, everybody seemed to have a great time. That was the best 'crowd' that I've ever been with when at some public event. It was awesome.