That’s not how casting works, though.
A reference variable is not an instance of the actor:
Think of the above variable as of a blank sheet of paper you can write a house address on.
Where’s the actual house?
In 99.9% of cases people do not have problems with casting, casting is trivial:
There, done! It’s just a single node that does the conversion.
People have problems with obtaining a valid object reference - called Actor Ref here. So, we’re looking for an instance of BP Door:
Which door? How do we know which one out of the hundred doors in the scene is the correct one? Once you know which Door Actor you want, you can store that info in Actor Ref variable.