This is another experiment into the Bystander Effect by Latane and Darley. A subjects was in a room either alone or with actors. Harmless smoke was then fed into the room and the subject's reactions filmed and timed. The actors, when present, ignored the smoke.
As with other experiments, the 'smoke-filled room' shows how the presence of inactive others can inhibit our own actions even to the extent of risking our lives. I like this experiment because of the way it brings out how extreme the Bystander Effect can be.