Double-Duty Rolls
Since NPCs don’t roll for their actions, an action roll does double-duty: it resolves the action of the PC as well as any NPCs that are involved. The single roll tells us how those actions interact and which consequences result. On a 4, the PC wins and has their effect. On a 2/3, it’s a mix. Both the PC and the NPC have their effect. On a 1, the NPC wins and has their effect as a consequence on the PC.