You need a physical stigmata that externalizes a character's hidden state: a mark, scar, or technology that makes emotion impossible to conceal and forces perpetual self-control.

Foyle's tiger tattoo is a built-in lie detector and control device: it resurfaces blood-red whenever he feels rage, terror, or possession, making his inner state legible to everyone in the room. Jisbella's warning is the price: "You've got to hold yourself with an iron grip." Bester turns the body into an enforcement mechanism against its owner, so Foyle's rage is always public and always costly. The face betrays the animal before the man can hide it.