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.