Small opening in set walls allowing actors to look through — adds depth and breaks the stage-front flatness.
You need a small opening in your set wall or decoration that your actor can look out of – this is the peephole. Not to be confused with the theatrical proscenium format (the stage as a picture within a frame). This is about a concrete architectural solution on set that creates sightlines and breaks up the flatness of your image.
Practically, it works like this: Your actor stands in front of a wall – a door, window, wall, whatever – and looks through a punched-out opening. This opening must be large enough to see their eye, but small enough to create curiosity or tension. This automatically creates depth staging: there's the foreground plane (wall), the middle plane (actor), and what lies behind. Your viewer follows the gaze outward – you draw them into the image instead of staring at them frontally. This is psychologically much stronger than if the actor were simply looking forward.
On set, you don't need much craftsmanship for something like this: a wall made of Styrofoam or plywood with a recess. The edges shouldn't be visible – so cover the depth of the wall or furnish it. Pay attention to the lighting direction: the light falling through the peephole must correspond with your illumination in the room, otherwise it will look artificial. If your actor is looking out through a window, the light must come from outside. If they are peeking through a small opening in a dark door, you need targeted, warm light from inside on their face.
Classic application: horror and thriller scenes. Actors at the peephole – tense, searching, unknowing. Or a romantic drama: she looks out, he looks in. This immediately creates visual storytelling without a line of dialogue. The peephole was also popular in early cinema's Expressionism because it enhances these geometric, restless compositions. Don't forget: the size of the opening determines how much of the face you can see – experiment during pre-production.