Once upon a time — say, around 1982, when he was strung out on heroin and singing as though a Ridley Scott-style alien had just burst through his chest — it was hard to imagine Nick Cave sitting in front of a fireplace talking quietly about his childhood with a chin-stroking interviewer. Like Iggy Pop, Cave has metamorphosed from rock's wild child to its elder statesman, but seeing how much he's mellowed out is still the biggest surprise of "20,000 Days on Earth."