Caption: The Alien in an elevator.
Features:
The Alien can determine the elevator's acceleration with the right equipment and from knowing the gravitational field.
Some examples elucidate the situation:
Everything in the elevator is weightless. The Alien is floating carefree---until the elevator hits the bottom of the elevator shaft, of course.
Really, the elevator has to push up on the Alien's feet with normal force m*(2g) to counter gravity on the Alien and accelerate the Alien upward at rate g.
Likewise internally, every layer of the Alien's body has to counter gravity on every layer above itself and accelerate every layer above itself at rate g.
So the Alien feels twice as heavy as normal.
In rocket jargon, the Alien is experiencing a G-force of 2g.
In an unaccelerated elevator, the G-force is 1g which is what we all normally experience.
Inertial forces act equally per unit mass on all bits of a body---just like gravity. For this reason, they are considered body forces