Child Animation Blueprint that inherits a Layer Interface can have the Layer Interface added a second time giving the impression changing the layers within the child will override them for the child.
In UE4 when you try to add Layer1 to the Animation blueprint it issues a warning saying it isn't blueprintable.
If this is intentional, it should be re-thought because its counter-intuitive.
Child Anim Blueprints should be able to override their parent's Graphs, even the main graph, and should be able to get parent graph evaluation like how a normal function can call its Parent function if desired.
Functionally you can override the Animation Layer Interface Outputs by creating a new class and giving it the interface, then calling Character-Mesh-LinkAnimClassLayers(separateAnimBP)
But this forces us to separate even more functionality that could be contained in the same Blueprint.
This is especially a problem when using Animation Blueprint Templates. Because the template may utilize Animation Layer Interfaces as a way to get the pose of a montage animation running on a slot (because slots don't work in templates).
The child class of the template can't implement their own version of the animation layer interface layers. There has to be another sister class that is not inherited from the template that implements the interface, and then is LinkAnimClassLayers to the mesh/childAnimBP.
While you're reading this. Hear that we want a way to get Montage Slot Poses to be used in BP Templates.
We want to be able to override Animation Graphs/Layers/Cached Poses of the parent/template Animation BP.
Either AnimBP_Child plays ThirdPersonIdle; or the layer Interface can't be added to the child; or adding the layer interface to the child overrides the parent.
ThirdPersonIdle doesn't play on AnimBP_Child; the layer interface can be added to the child.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-147316 in the post.