At a high level, the BP interface itself just defines the function names and parameters.
Then you implement that interface in any class you want.
Finally, this enables you to call that defined function given any object.
If that object supports that interface, the function is called.
Otherwise, it does nothing.
So the main benefit is it lets you design without needing a specific type.
In your case, you can avoid any casting and just call the function on the Actor from Overlap.
One more thing - make sure that your collision channels are correct and the overlap event is indeed firing when you think it should.
To test, put a print string right after the overlap event.