Foil

A foil is a character placed beside another to make that character legible by contrast — either by being their opposite or by being an exaggerated version of them.

The jeweller's term is the origin: a thin sheet of bright metal set behind a stone to make it shine. A foil is not necessarily a rival and not necessarily an antagonist — it is a mirror held at an angle.

Two versions do most of the work. The first is the opposite: a sibling, partner or sidekick who differs in every way the protagonist cannot see about themselves, so each choice the protagonist makes registers as a choice rather than as the only option available. The second, and often the better one, is the exaggeration — the same person, turned up. Frasier and Niles Crane work this way. Niles is not Frasier's opposite; he is Frasier with the snobbery amplified past deniability, which lets an audience see in one brother what it would forgive in the other.

The revision test is whether the pair produce different answers to the same question. Put both in one scene, hand them a decision, and if you cannot predict two distinct responses, you have two people with one voice — and a line edit will find them sounding identical in dialogue as well.