Preflight, Interrupted
The airline industry long ago figured out that one of the most dangerous things in aviation is two pilots trying to fly the same airplane at the same time. One inevitable result of such an arrangement is that there are times when no one is flying, and one of the ways we know this is from the accident record. Airlines evolved the pilot-flying/pilot-not-flying concept to acknowledge this characteristic of crewed cockpits and established clear responsibilities for each pilot.