Well I do perform this effect for people, and I have to say the hardest thing about it is trying to explain to the spec whats about to happen. The majority of the time they flip out when the top card changes to the one you placed in the center outjogged. They don't even realize that they switch.
Put in the time for a well thought out patter and you'll have your self a real fooler.
As for a story... I once did this effect right after Roy Waltons Time Machine cause of the similar plots. As I explained to the spec that "This time I'll leave the card on top face up". As soon as I changed the card I didn't even have enough time to pull out the out jogged card she spilt her drink all over my Tally's and kept saying "No [censored]' way".