Foil
A foil is a character placed beside another to make that character clearer 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. Shakespeare plays on it in Hamlet, when Hamlet tells Laertes before their duel, "I'll be your foil, Laertes." The joke is that Laertes is already Hamlet's foil in the literary sense. Both have lost a father to murder; Laertes rushes to revenge at once, and Hamlet delays for five acts. Each makes the other's choice visible.
Two kinds of foil
A foil is not necessarily a rival and not necessarily an antagonist. Think of it as a mirror held at an angle.
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.
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.
Foil examples from The Runner
The Runner, a feature screenplay by Jeffrey Doka, the editor behind Final Boss Editing, puts two foils in the first page of the cockpit.
RICH DAVISON (32), a lean, neat pilot-type, sips coffee from a worn thermos and adjusts the collar of his leather flying jacket. Giving his shaggy, handsomely scruffy copilot a tired look, he checks the airspeed--holding 150.
Rich's copilot, JAMES WILLIAMS (25), finishes his yell. With a contented sigh, he slouches back and watches Rich work.
RICH
Save it til we land.
Why it works: neat against scruffy, coffee against a rebel yell, work against watching. James's ease makes Rich's control, and his tiredness, stand out in the first lines.
The script then builds a second pair on the same frame: an F.B.I. pilot, Sandra, and her copilot, Robert. Both pilots take a dive their copilots warn against. James shouts "Stop! You'll stall us" (page 5); Robert shouts "Dive's too steep. Level out!" (page 6). Rich pulls out of the dive; Sandra's plane loses its rear windows and has to make an emergency landing. Set side by side, the two pilots show what Rich's recklessness is worth: the same gamble, a different pilot, a different result.
How to test a foil
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.
Frequently asked questions
What is a foil in literature?
A foil is a character whose contrast with another character, usually the protagonist, makes that character's traits easier to see. The foil can be a friend, a sibling, a rival or an enemy.
Is a foil the same as an antagonist?
No. An antagonist opposes the protagonist's goal; a foil contrasts with the protagonist's character. One character can be both, but a best friend or a sidekick is more often the foil.
Where does the word foil come from?
From jewellery. A foil was a thin sheet of bright metal set behind a gem to make it shine, and a character foil does the same job for another character.