Theater (stage) and movies are related in many ways, but what are the differences that make them different art forms?

One of the differences between movies and stage
productions is that stage productions are all-encompassing.  Everything is live, and
everything that is needed is right there, right then as the actor is acting.  In a
movie, scenes can be refilmed over and over again in different moods, and directors can
edit them later.  Therefore the art in theater happens all on the stage, and in movies
happens on the stage and in the editing room.

Another way
movies and plays are different is in the technical aspects.  Movies and plays both use
lighting and sound effects.  Both can also have special effects and stunts, but the use
of these is far more limited on the stage.  Therefore movies are not just made by the
acting, but rather an intricate and elaborate combination of all factors carefully but
together.