Sia’s greatest songs, like “Unstoppable” and “Titanium” and “The Greatest,” are anthems of strength, power, fortitude, invincibility, and watching the movie it’s fascinating to behold how her facelessness feeds that quality. She’s an abstract figure, a pair of lips singing, and this creates the sensation that the power she’s singing about doesn’t emanate from her. She’s channeling a force larger than herself — the rising specter of feminine triumph. She’s merely the vessel.