Creating an Actor Variable does not automagically create an actor instance 
That variable is just a reference that could point to an actor of that type - think of that variable as of an actor’s address. Where is the actor, though? How did it make it into the game? Which actor instance in particular do you mean? What if there’s 12 of them? How do you want to choose which one?
Consider spending some time with this:
As in the post above, you need to assign value to a variable for it to be valid. Once this is done, you may not need to cast at all…