It doesn't need to be literally their hand through their hair, it just needs to be some action which is easy to perform but complicated to photo-realistically s

It doesn't need to be literally their hand through their hair, it just needs to be some action which is easy to perform but complicated to photo-realistically simulate in real time from an arbitrary starting condition. Have them tug on their clothes to see how the fabric moves, have them or a caretaker turn a nearby light on and off such that their illumination changes, etc.