Lets say an automatic transmission vehicle is going downhill and stopped in traffic. And suddenly engine died. Is it possible for the wheels to turn (turning the transmission, torque converter) and vehicle to go forward?
If the engine stops while in gear (and brakes not pressed) it IS still possible for the vehicle to roll or continue rolling. Brakes will require additional foot pressure to maintain the same amount of braking power.
If moving: The wheels will continue to rotate, however on some vehicles it will create an 'engine braking' effect and the resistance of the engine will slow your car down. This "engine brake" effect does not apply to modern automatic transmission vehicles. They will continue to roll without resistance.
Just a note: You'll also lose power steering and 'powered' braking. If key is turned to full "off", the steering column will lock too. However, you cant turn key to full "off" while in-gear.