Serious Eats - seriouseats.com

Here in Japan I eat lunch at a restaurant whose owner's son in law appeared as a challenger on the original Japanese Iron Chef. He told me that the participants were given a list of potential ingredients (either 3 or 5, can't remember now). So they did have the opportunity to do a bit of preparation, even though they couldn't be sure what the show's them would be.

He also said that the staff would give the challenger chefs advice if they were running short of time, such as letting them know they could skip the platting and serve the food to plates at the judging table.