We did a mad scientist routine once. The basic schtick was that the 'monster' (the tallest guy we knew) would lay on the table while a guy in a lab coat worked on him. I played the nurse (with the astoundingly original name of Ratchet), and helped in the patter - which we actually changed up as the groups went through. At some point, there would be a phone call ('Doctor, it's Life magazine about your subscription.' 'I don't subscribe to Life, I create life!' / 'Doctor, it's the White House. They want you to return Dan Quayle's brain.' 'Why? He's not using it - it's far more useful here.') He'd have one of the attendees look through the microscope - sometimes say something about how what should have been on the slide must have escaped and infected them, or crow about finally accomplishing invisibility.
Eventually, the monster would sit up, roar, and tear away the mannequin arm that the doctor had been concealing up his sleeve. I would scream, and the ushers would hurry the crowd to the next room.