In a wonderfully honest interview from 1981, a young, dark-haired Ian McKellen elegantly explained to talk show host Dick Cavett the difference between acting for stage and acting for film. McKellen specifically focused on body movements, stating that the distance between the stage and the audience requires more extreme positioning than up-close cameras. Additionally, the camera can show the audience where to look, whereas onstage, that directional focus is missing.
The trouble about the stage is that the audience sees the whole body. They are far enough away from it, even in a small theater, to take it all in and of course they because they’re not engaged in the action of the play. They are there only to be an audience who listen and indeed a spectator who watches. They can take it all in with an objectivity which normally they don’t apply …the crucial difference that the camera is very like somebody just in the room with you. It concentrates very partially on you, usually on the face, and the body can be doing what it wants.