The Monster Maker — Ray Bradbury

Two men search an asteroid for a den of space pirates; one is a huge hulking Irish space cop who says stuff like "Me father taught me; keep laughing and you'll have Irish luck," and the other is a cameraman obsessed with movie making--when he fears he is about to die in a space ship crash he asks the Irishman, "Is this where the Big Producer yells CUT!?"  

A horde of monsters appears and chases the two men, who take refuge in a cave; when the monsters don't show up on the photographer's quick-developing film, they realize the creatures are just illusions.  They sneak into the pirate's base, get control of the machine that generates the illusions, and trick the pirates into thinking an entire army of Irish space cops has arrived.