casting is pretty simple once you understand what its about. a cast is basically just comparison, asking is this object similar to another know object. imagine you have a car and you dont know the make and model, a cast would be like parking your car next to a mustang and asking are they the same. thats basic casting. now once you get into inheritance things are a bit different, lets say your looking for something of type car meaning 4 wheels and same basic shape. now if you input a mustang it will be true but if you input a plane then it will fail because a plane doesnt have the basic attributes (doesnt inherit) from car.
in your case though you could probably have just saved your reference in the game instance or game mode for easy access and so you dont have to keep setting the reference.