Jason S. answered 10d
University Voice Professor | Professional Singer & Actor
The fundamental work of acting is the same in both film and theatre: listening, responding truthfully, understanding what your character wants, and being fully present with your scene partner. The biggest differences are in how that work is communicated.
Onstage, an actor has to communicate across a physical space, so the voice, body, and physical choices often need enough size and clarity to reach the entire audience. On camera, the lens can capture extremely subtle behavior, so a thought or small shift in expression may communicate everything the audience needs. Film also requires actors to work out of sequence, repeat moments across multiple takes, and adjust to different shot sizes while maintaining continuity.
I don’t think of these as two completely different kinds of acting. The truthful behavior underneath should remain the same. The skill is learning how to adjust the scale of that behavior for the medium without sacrificing honesty or connection.