Left side from viewer perspective — actor's right. Standard spatial reference for set directions and cutting instructions.
On set, spatial orientation is a fundamental requirement—and Screen Left is adopted here without much thought. You're talking about the left side of the monitor, the screen, the image from the viewer's perspective. So the actor is actually standing on their right side—but that's irrelevant. Screen Left is the convention everyone understands: camera, director, sound, lighting, editing.
Why is this fundamental? Because it creates continuity. If your protagonist is sitting on Screen Left in shot A (i.e., on the left side of the frame from the viewer's perspective), they must still be sitting on Screen Left in shot B if you're looking across the 180-degree axis—otherwise, they jump over, and the scene feels disjointed. This is called axis continuity or eyeline match. Without this common language, editing errors occur, which are often only noticed during color grading or even in the cinema.
On set, you say: "Light on Screen Left, Actor A is sitting Screen Right of the couch"—and everyone knows what's meant. The focus puller, the assistant director, the gaffer—everyone works with the same visual coordinate system. This saves you misunderstandings regarding the positions of props, camera movements, or light modulation. Unlike "right" or "left" (which are always relative to the respective viewpoint), Screen Left remains absolute—from the viewer's perspective.
In editing, it becomes even more critical. You juxtapose two close-ups: an over-the-shoulder scene. If both actors are looking towards Screen Left, the scene appears disturbed, as if they aren't looking at each other. The vectors of their gazes don't cross. This is the second rule after axis continuity—the eyeline direction must be consistent. Screen Left and Screen Right are your only objective terms to discuss and verify this afterward.
Good advice: Make a note during scouting or storyboarding about which action happens on which side. A simple floor plan with Screen Left at the top and Screen Right at the bottom—two minutes of work that will save hundreds of hours in editing later. It's the basic language between the director and the camera, and those who don't speak it fluently lose time and money.