I see people asking for full AI or whatever, but I agree that more thorough reactions or a larger volume of responses would be the easiest way to make the characters seem more alive. Making them walk around and interact on their own seems like feature creep hell.
Yeah and i dont think that really adds much to the gameplay.
I mean what do you gain from needing to find Hera on the beach map because she decided to wander off? Your main gameplay will always be posing her for sex, what extra do you gain from playing fetch with their spawns?
Honestly the system in Secret Horse Files where each character has feelings towards specific toys, other npcs and specific actions sound more like the system you need here for immersion.