Extras, Ricky Gervais' comedy about a middling actor's struggle for fame, is useful for many reasons. It gives us the opportunity to see Kate Winslet giving sex advice while wearing a nun's habit, for one. It allows Daniel Radcliffe to work blue. And it gives the public a glimpse into the songwriting process of David Bowie, a man who can find inspiration in any face, no matter how fat, and who, evidently, is never more than an arm's length away from a piano.