Whispering in someone's ear is perfectly acceptable. Heck, get emotional! Cry, laugh, shout in terror, but don't carry a farking conversation every 10 minutes. I paid for a movie...
Last thing is first. Everyone paid for the movie (well, maybe not all the teenage derelicts).
I think most would agree with my rules of theater etiquette.
Conversations are a definite faux pas, as is the vague question "what just happened?". "Where do I know that guy from?" however is a valid question as when asked in your "theater voice". The response to the this question however can be said in slightly louder than library voice, example: "That's the villain from Billy Madison".
If the entire audience is loud, participate. I remember watching RUSH HOUR and wondering if anyone, in the entire theater would ever stfu. Then I relaxed and joined in on the mindless fun.
If you have a solid joke, it's fine to say it out loud. Once and only once during the entire film. If you have the unpleasant fortune of sitting next to the guy who thinks he writes for MST3K, a simple "You've done enough" should get him to shut his pie hole.
Mobile phone off.
If you bring in your own food, make sure it doesn't have an odor other than candy or popcorn.
Please don't throw gummi bears at the screen. Yes, they stick, but the same screen that you threw gummi bears at may be the same screen that shows This is 40.