He turns up mid reception in workwear with a ladder over his shoulder, entirely unbothered, insisting he has been booked to look at something. Nobody knows anything about it. He starts setting up anyway.
There is a bit of back and forth about paperwork, a few loud complaints about the parking, and somebody from the venue starts moving toward him. Then the drill goes down, the microphone comes out, and the builder nobody wanted in the room turns out to have a voice that stops the entire party dead.
The scruffier the setup, the harder the reveal lands. From there it is a full show, and your guests spend the rest of the night working out who was in on it.