You can't move in London for American performers, whether it's the Yankee contingent of The Bridge Project at the Old Vic, or the presence at various addresses of Mercedes Ruehl, Jeff Goldblum, Glee star (and erstwhile Tony nominee) Jonathan Groff, and, of course, pretty well the entire cast of Hair. But incomplete though that run-down is (one mustn't forget the silvery voiced Sierra Boggess in Love Never Dies or David Hyde Pierce's stern-faced mien in La Bête), few visitors have fired up the public as has Whoopi Goldberg, at the Palladium for three weeks to boost the musical, Sister Act, on which she also gets top billing as producer. And how is the Whoopster in a wimple? The more fascinating topic is the galvanising effect that her arrival has had on the show as a whole.