Negative fill

Every room bounces light back at whatever you are filming. Pale walls, a white ceiling, a desk surface, the sheet of paper you are reading from, all of it filling in the shadow side and flattening the face. Putting something black close to that side absorbs the bounce and lets the shadow go dark again, which brings back the shape a soft key light washed out.

A proper flag is a black cloth on a frame, though a piece of foam board, a folded blackout curtain, a black t-shirt or a coat over a chair does the same job. Move it closer for a harder edge and further away for a gentler one.