Input the reference of object from blueprint that variable you want to access in to “Target” input. “Target” input is always for object you want to operate on, in nothing is plugged to it, it defaults to “self”. To get that node from object you need to hold the link from that object and drop it in empty part of blueprint so it shows all possible connections, in “Variable” category you will see get and set nodes of all public variables of object.
You don’t need interface you can just cast, interface is for relate unrelated classes not for communication which lot of people misunderstand,
Here you got my video which explains class relations and inference and also little about interactions of blueprints