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Do you think it's just a stage vs camera thing? The main difference is that a theater actor has to project everything - voice, gestures, emotions - toward an audience that's at a distance, while a film actor works for the camera so even a whisper or a glance is enough because the camera picks up every tiny detail.
In theater you have to project everything toward the audience that's at a distance, so you need a more powerful voice and bigger movements, whereas in cinema the camera captures you up close and a minimal expression is enough to communicate. The shooting itself is completely different too: in theater you do everything in one go and the audience sees it all in real time, in cinema instead you can shoot scenes from a hundred different angles and redo a scene a thousand times until it's perfect. So basically, theater requires a certain "construction" of the character that's visible from far away, cinema is more natural and intimate.
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