Catherine Zeta Jones is a remarkable actress and that is what they do: act. They don't necessarily have to master the role that they are playing. It does not mean that they don't practice since the more knowledgeable you are about the role, the better you can act. Naturally, for "No Reservations," she actually learned how to cook and what chefs do in real life. Here is a video interview of Catherine Zeta-Jones about the film and how she prepared for it.