How has chillingworth become "[not] a spectator only, but a chief actor, in the poor minister's interior world.?"

Chillingworth has pretty much figured out that Dimmesdale is the father of Pearl. He can see Dimmesdale's own torment and becomes a participator and instigator in Dimmesdale's self-destruction. Chillingworth becomes Dimmesdale's "personal" physician slowly poisoning the priest's mind and body.