In the documentary 20,000 Days on Earth, Australian rock icon Nick Cave says his work has always been focused on building his own world — one he calls absurd, crazy and violent — through song. Cave, 56, became a rock star as leader of The Birthday Party and The Bad Seeds despite just one fluke hit, 1986’s “Where the Wild Roses Grow” with Kylie Minogue. And a rock star has to be godlike, he says, adding, “It’s all an invention.” In the film, the disparate elements of that invention are compellingly revealed, in a rhythm that’s slow but stimulating, much like Cave’ s bluesier work. The movie’s innovative approach to storytelling starts with a dazzling 150-second credits sequence that chronicles the first 50 years of Cave’s life. For the next 90 minutes, he is seen interacting with his late father, writing and recording with his band and a children’s choir, and reliving his past in chats with former bandmates, a psychiatrist and Minogue. Putting Cave in a series of intimate situations gives 20,000 Days on Earth its unique flavor, but it’s his openness and intelligence that makes it sing.