SETUP: Create a Blueprint Interface asset (BPI_Interactable) with a function 'Interact'. IMPLEMENT: In the actor BP, go to Class Settings → Interfaces → Add → BPI_Interactable. Then implement the 'Interact' event that appears in the event graph. CALL: Get actor reference → 'Does Implement Interface(BPI_Interactable)' → Branch → True → 'Interact (Message)' node (the interface call). Key: Interface calls are 'Message' type nodes. Use 'Does Implement Interface' before calling to avoid crashes on actors that don't implement it.