Upload your original video footage to your computer and import it into your chosen computer program. This will act as the background for your theater seat silhouette's footage.
Hang a white bed sheet from the top of an open garage or window. This will act as your screen, helping you to display your actors in silhouette.
Sit your actors down on chairs on the outside of the sheet. When you sit on the inside of the house or garage, the sun shining through the sheet will leave your actors unseen apart from their darkened silhouettes. Keep the actors as close as possible to the sheet to ensure that their outlines are crisp. Film the scene required by your script.
Import the new footage into your computer program and remove the sheet from the frames, leaving you with a black silhouette. This can either be accomplished with a filter or a cutting tool. Import theater seats if your program does not have them already installed.
Find the beginning of the sequence where you want the silhouetted footage to begin. Add a new layer to your original video footage and place the new silhouette footage into that layer. Position the silhouette along the bottom of the original footage.