When going to someones home or even your own home for an at home movie night the same rules apply to be quiet during the movie. Obviously this rule will be a lot more flexible since you are at a home and not a theater but some people are really into movie and want the full experience of not being completely disturbed and to really focus on the movie film plots.
If you find yourself engaging in conversation while the movie is on ask that person or persons to go into another room or away from the movie watching area to finish that discussion. In fact as long as your keep your voices a little more low than usual then everyone can play catch up and everyone else can witch the movie it is that simple.
Also do no become a commentator, or movie jokester, not everyone thinks you are should be the center of attention. The only times that the commenting rule is more flexible is when there is a movie on that everyone has already seen and now all of you are part of the entertainment but other than that there is no reason anyone should have to put up with and arm chair movie quarterback.