Guile is deliberate deception: convincing a listener of something the speaker knows to be untrue. It is close kin to Theatrics but differs in what is being falsified — a character using Guile misrepresents the facts while remaining entirely themselves, where a character using Theatrics misrepresents who they are.
It covers bluffing — a litigant arguing a case they know to be weak, a friend steering an ally toward an action by leaving something out — and outright lying — the merchant selling false wares, the thief explaining themselves to a town guard.
Resistance. Deception is resisted with Intrigue. It is worth being precise about the difference from Empathy: a successful Empathy test reads emotional state and no more, so a character may correctly perceive that someone is nervous and still have no idea they are being lied to. Intrigue is what draws the second conclusion from the first.
Where a character lies while also disguised, Guile is the skill tested and Theatrics lends its Secondary Modifier.