Be yourself, always be yourself. Don't ever try to pretend to be more or less than who you are. If they ask you a question on dealing with a situation, say how you would actually deal with it, not what you think they want. You have to go home at the end of the day and be happy with yourself, and not being yourself is a good way to get miserable quick.
As far as cooking, cook what you know. Have a few main ideas of what you want to prepare and make sure they're versatile and have room for adding or subtracting in lieu of what the kitchen has available. Most places I've had to cook for usually make me cook them an entree, which they expect to see good skill and flavor composition, and then to cook them something simple, like a burger or an omelette. The catch to these cooking exhibitions for your interview usually lay in these simple things. Put as much thought into that burger as a structured entree. I've seen kids not get the job because they didn't season their burger before they cooked it. Chefs want to see people who will give their all whether it's a $35 entree or a $10 burger.
