I think if I was a doctor there would be a strong desire to tell someone they have a deadly cancer just to see a reaction. The issue is that they could take it the wrong way. They could get really upset that you tricked them like that (because it’s their life, and everything), so you’d have to do a bunch of little surprises to see if they could handle it.
Maybe they come in for a checkup, and you say, “I’ve got terrible news: you have a wart on your hand.” When they say, “What are my options?”, you blurt out, “You don’t have a wart!” Do this six or seven more times with other minor conditions. As a doctor, you should already have a list of these minor conditions.
On a related note, never believe anything your doctor tells you. He or she might be using my technique, and you don’t want to be taken for a fool. The doctor will respect you more if you question his or her every action and statement.