You can have a lot of fun getting creative with this one in a bunch of ways. The first and easiest is to wait until your friend leaves the room, then turn every single picture on the walls, sides and everywhere upside down. Better yet, bring along a bunch of completely random photographs and slowly but surely begin replacing those in their room, one at a time until they notice. And if it happens to be your place they’re coming to, maybe replace every picture in your own living room with the exact same weirdly serious photo of you…or of them.