A point has come in your screenplay where the lead has crossed the threshold into insanity. Now it is time to construct the padded cell. The iconic padded cell is a white-washed, small space with padding on the walls, door and the floor. The California Code Regulations of a holding cell specify ceiling heights, toilet system, surveillance equipment, which may be helpful in creating the layout for your cell.
Select a small space with no windows and fairly flat walls for your cell. A large, shelfless closet or empty basement room are both good options. Consider the exact specifications for actual "safety cells" in detention centers if your film is dealing with a realistic setting.
Cover the walls with cotton padding. Hang from the top of each wall, stapling or nailing the padding as flush with the ceiling as possible. Staple the bottom of the cotton as flush to the floor as possible. The cotton should "puff out" in the center, this will give you a padded wall effect.
Cover the cotton with your white sheets or fabric. The sheets should be pulled taunt until they are completely smooth. Staple or nail the sheets in the same way you stapled the cotton.
Paint the floor white. You may choose to cover the floor with padding and sheets as well, but as the actor moves this may cause residual noise and cumbersome movement.
Hang your lights. A single bulb hanging from a wire in the ceiling can add some creepy shadows. The white walls and floor will be very reflective, you may want to use this to your advantage and do a white-washed shot that will make the actor stand out.