Film and stage are quite similar in that they both are looking for truthful behavior within imaginary circumstances, or to quote Stanislavski, "Actors must live privately in public" regardless of medium.
Unfortunately there is a lot of misunderstanding of both delivery mechanisms that leads to the belief that the two are very dissimilar.The difference lies in the technical demand of each and in the way in which audiences view the material. I am just adding here to many of the comments that have gone before.
On stage, the actor must draw focus; on camera the camera focuses on whatever the director wants to see.
On stage, the actor has the ability to repeat a performance night after night which usually deepens their per and therefore the emotional wallop of the play. On camera, the actor repeats a scene or a sequence over and over, which should, if the actor and the director are in sync, produce different and or better work.
On stage, the actor does a scene and goes off stage for only a …