There are some rules, or as I say, 7 Commandments which I follow while watching a movie. It is my courtesy towards the movie.
1. I watch any movie from the very first second of its beginning, until the very last moment until its credits get over. Leaving the movie when credit BEGIN to roll, can also be allowed, but is not preferred.
2. I never multitask, while watching a movie. Thats because I have to catch every frame of  it. Technically its not possible (cause we have to blink!), but virtually it is.
3. I remain far away from Spoilers as much as possible until I watch the movie. And I hate when someone spoils it for me. (Then I become angry!)
4. I may read about the plot, and watch its trailer before I go for a watch, but that’s only to maintain the excitement and enthusiasm. And so that I may understand the story better. But to the extent that it doesn’t violate Rule No. 3. (Read above.)
5. I prefer to watch any movie in its native language with subtitles, rather than dubbed. Reason is simple – to listen to the original sound, it contains the level of enthusiasm, which can’t be replicated in the dubbed version.
But this rule is not strict. And I sometime break it, when I sense that movie is going to be complex, and more verbal.
6. I hate when someone breaks the aura of moment between a sensational movie scene, with the ring tone of his mobile, or to explain his theory over what just happened, or to predict what IS going to happen, in an attempt to show his smartness!
7. Rules 1 and 2 are of utmost importance, and shall never be broken by me, nor do I expect anyone else to break when the one is watching a movie with me.