Tape or chalk on set marking actor positions — ensures continuity and lighting consistency. No line, no reliable match between takes.
Nothing works on set without lines on the floor. The camera assistant or the assistant director tapes — usually fluorescent gaffer tape in neon colors — precisely onto the floor where an actor stands, sits, or begins a movement. These lines are the invisible scaffolding between takes. They record: Where was the main character during the third camera pass? On which line was she standing when the lighting was perfect?
Without these lines, chaos ensues in the edit. The editor later sits in the cutting room and realizes: The actor's eyes have shifted 30 centimeters to the left between two shots. The elbow is suddenly too close to the body. The light coming from the window is now on the wrong side of the face. Continuity breaks down. An entire scene might have to be reshot — or remains unwatchable. So, the lines are not pedantry, but craftsmanship.
Practically, this means the assistant meticulously observes the rehearsal. Actor A stands three steps away from the door frame, foot on the middle floorboard. Two short strips of tape — one under the heel, one under the toes. Actor B sits on the sofa, right arm on the armrest. Another line. A focus puller working with the focus needs these lines just as much as the 1st AC — the focus must be pulled precisely, and that's only possible if the actor maintains the same position.
Colors have a system: Gaffer tape in white for the main character, red for supporting characters, sometimes the takes are also numbered. Movement lines are more complex — here, an arrow is often taped to show where the person should walk, or multiple points are set (start, midpoint, end). The actor quickly learns to pay attention to these lines out of the corner of their eye without looking directly at them.
With Steadicam, dolly, or complex camera movements, it gets even more critical. Any deviation from the line shifts the composition. Therefore, the choreographer and the director work closely with the DOP before the set — the lines are the physical proof that the actor is adhering to the blocking instructions. Without them, continuity is a matter of luck.