To say an actor is an actor no matter which stage they're on is discounting the different set of skills needed to act on a stage rather than on a set.
Stage actors must emote, project their voices and move about the space so the audience can see what they're supposed to on stage, and they don't have a hundred takes to get it right.
Get the difference? If not, consult this clip from Conan where screen actor/master thespian Timothy Olyphant demonstrates the different in a most peculiar way. (Barely NSFW due to language)
I'll never think of the phrase "pan out" the same way ever again!
-Via GeekTyrant