8 Simple rules for watching a movie in a shared living room:
1. DO NOT walk in on the movie halfway through, and continually ask the person watching it to tell you what's going on. JUST WATCH THE FREAKIN' FILM! The person will not mind if you do so, at least not as much as she minds constantly pausing to explain. This goes doubly so if the person watching the movie has never seen said movie before.
2. DO NOT constantly ask what's going on when you've been watching the movie from the beginning. More than likely, the film itself will answer your questions--that's kind of the point. If you don't understand after it's over, then you can ask questions. If you just want a few sentence summary of the film, go read IMDB.
3. DO NOT constantly come through the room, talking loudly or making a lot of noise. The person watching the movie wants to be able to hear it. If you find that they are continually turning the volume up, you are being too loud. (NOTE: This only applies if the person watching the film was there first. If the person talking was there first, the movie-watcher is being rude and gets what she deserves.)
4. DO NOT mock the movie if you notice that the person watching it appears moved by the story.
5. DO NOT talk about bad effects on classic movies. Nobody cares. All past effects are bad by the most recent standards. The Red-Sea-peeling-back on The Ten Commandments is still cool, regardless of looking a little funny.
6. DO NOT mock the acting style in classic movies. A different acting style was in vogue at the time; they would have considered today's actors terrible. Just get in the mindset of the time. It's not that hard.
7. FOREIGN FILMS AND ART FILMS are just weird. Do not ask the person attempting to watch them to explain it. Even if they like it, they won't be able to explain why. If you don't get it, just accept it, and be quiet.
8. IF you don't like the movie, just leave. Don't mock it, complain about it, or try to drown it out. Go read a book. (Again, only applies when the movie-watcher was there first)