While a cast member is our filming a prank, perhaps over a weekend, go into their house or appartment and turn everything upside down. By that I mean permanently fix all the household furnature (ie. desks, bookshelves, bed, fridge) to the ceiling. So he'll return to find his world had been turned upside down... literally.
Hang any books up to the shelves with fishing line and staples to give them the gravity defying effect of floating books.
Wardrobe, vases, clothes... hang absolutely everything you can possibly find in their house in the same position it normally is, but upside down hanging from the ceiling.
Film their reaction when they walk in the door.
I thought it would be a clever idea.