Think about it like this:
You want to send a message 
to a specific actor instance 
that has this function implemented via an interface:
Which instance of the actor would you like to send it to? What if there are 10 dudettes in the scene and their eyes need fixing up? How are you choosing who to send the message to?
It does not matter how many instance there are, even if there is only one - you need a Target - that’s the pin.
People never have issues with interfaces or casting ← this part is trivial, it’s just one node. They have issue with referencing - finding which actor they want to talk to.
How to approach things depends on the job at hand. For example, if you’re possessing a pawn and want to communicate with an interface, you can just:
There can be only 1 possessed pawn at a time so this will always work (unless you destroy the pawn, ofc - then it will fail quite gracefully). As mentioned above:
You need a target to send the message to.