Actors are like athletes. They need to run on the open track in order to be ready for race day. All too often I’ve heard young actors ask what “stage acting” has to do with the career they’re seeking in film.
My tendency is to turn to sports training for metaphors in cases like this, so if you’ll indulge me…. imagine if you were a swimmer doing strength training. There would be a number of activities in your regime that bear no resemblance to your sport. A swimmer can train on weights, for example, and never get wet. The benefits are undeniable, as long as the athlete continues to integrate training with application.
There’ss an assumption that stage acting is “too big” or theatrical. But this is only true if you act on a stage exclusively. Film acting is small and delicate, but has to be as deeply felt and grounded in your emotional life as King Lear’s speech on the stormy heath. The camera will pick up every little lie and will reveal if you’re not completely engaged. Just think of your worst photograph. Chances are you were disconnected and not very genuine.
It’s in the arena of great theatrical writing that an actor develops emotional depth and a size of personality that carries over to camera work. It gives you the skills to inhabit the shape and arc of a large role, learn repeatability and (when requested) improvisation and variety over a number of different takes.
You can really get “race-day ready” by having long run with a meaty role. For inspiration, here’s a great article from the New York Times.