To instruct people to pronounce the word 'cheese' so that their mouths form a smile, typically when taking a photograph; by extension, to prompt someone to smile.
Examples:
EN: - Should I say cheese?
ES: - ¿Debería decir queso?
EN: I'll say one, two, three, then everyone say cheese.
ES: Diré uno, dos, tres, y entonces decís todos "patata".
EN: - Big smile, say cheese. - Yeah. I thought so.
ES: Entonces este graffiti es tuyo otra vez.
EN: - All right, everybody say cheese.
ES: - Muy bien, todo el mundo diga queso.
EN: Now, remember to say cheese.
ES: Listo.
EN: All right, say cheese.
ES: Está bien, sonríe.
EN: Lucy, Marco, say cheese!
ES: ¡decid patata!
EN: If this is his camera in here, I want to get it and say cheese... so he knows that we're onto him.
ES: Si esta cámara es suya, quiero sonreír delante de ella... para que sepa que le vigilamos.