Place it at the exact cut where the hidden person steps into frame, after building tension with a quieter setup beat. Pair it with the Metal Gear Alert Sound right before the reveal to signal someone has been spotted, then let your next scene play without another effect competing.
About this sound
This is a 1.3-second voice clip of Sergeant James Doakes shouting the line "Surprise, motherfucker" with abrupt, confrontational energy. It feels cheeky and startling at once, turning a normal entrance into a comedic ambush that lands instantly. The clip comes from the television series Dexter, taken from Doakes' memorable confrontation scene.