This was the first "screen test". Based on one of the oldest and simplest "special effects" in the history of film-making, this experiment was shot completely backwards. The resulting footage was then reversed so that action appears to proceed forward in time.
Hans decided to explore the effect of creating twins on screen. This first test creates a simple division of the frame into two parts, allowing Murray and Murray to exist simultaneously. This scene took no more than 15 minutes to shoot.
This was shot immediately following the first twins test. This was slightly more complicated because the characters interact with the same setting.
Careful establishment of scene partitions allow the "ghost" and "invisibility" effect. This scene was a quick 5 minute improv.
The first forey into blue-screen masking. This was shot rather impromptu. Hans had a light blue bed sheet that he dragged out. As you can guess, this performance was a complete improv.