When carrying a body up stairs in a plane that is falling, less effort is required compared to when the plane is flying at a constant altitude or ascending. The key factor is the acceleration experienced during these scenarios. In free fall, similar to conditions in a satellite or during parabolic flight (like the vomit comet), the effective weight is reduced to zero, making movement significantly easier. Conversely, if the plane is ascending or maintaining altitude, the additional gravitational force must be overcome, increasing the effort needed to move the body.