there is a gereral misconception about "playing roles"
you can´t play what you don´t know (this is what you see as bad acting)
you have to have a parralel experience that you can draw from (the closer the better)
no actor "acts" out of nothingness, it is merely the ability to recreate feelings emotions behaviour that you experienced before.
the only difference is that the story your "acting" out is not "real" in the sence that it already happened.
Sci fi or fiction is nothing more then a recreation and different combination of things that you already know.