sadly i don’t really get how you are trying to use the Interface. But i will just generally explain the use of an Interface, maybe that already sorts things
An Interface can have multiple Function Declarations/Implementations. Declaration means it just Inputs and Outputs without Logic. Implementation would also have some Logic in it.
How and for what do you now use the Interface?
As you already found out, you can add Interfaces to BlueprintClasses. When done, you can OVERRIDE the functions of the Interface in this Blueprint and let the Blueprint give this Function its own Implementation.
Let’s say we have an Interface with a function called “Use”. This function has no inputs or outputs, to make it easier to follow.
Now you add this Interface to a door Blueprint. The DoorBlueprint now overrides the function and adds some opening logic to it.
Now you add this Interface to a second Blueprint. This one is for example a weapon pickup. It also overrides the Interface Function with a logic that makes you pickup the weapon.
And now? Imagine you have a line trace when pressing your E key. You trace the Door and the Weapon. Both will be returned as an “AActor” in the HitResult. Normally you would now need to Cast the AActor (HitActor) to the Door and call the “Open” function on it, or to the Weapon and call the “Pickup” function on it.
With more and more different actors you would need to cast to a lot of different classes to test if the actor you traced is of that class. This is kinda inefficient and would result in a really big blueprint function.
BUT that’s where the interface comes in handy. You can use blindly call “Use” on this Actor. IF the actor has the Interface and the Use Function overridden, it will call it’s implementation.
So you don’t need to care about casting, you just call “Use” on the Actor and if you hit the Door which has the Interface, it calls its own version of Use. Same for the Weapon and all other Blueprints you add the Interface to.
Actors that don’t have the Interface/Function will just ignore the call of course.