Quite a bit of This Much I Know to Be True happens in a soundstage. Inside that soundstage is what can be described as a literal circle of light. Within that circle, Nick Cave is the somewhat reluctant center of attention. Reluctant because even though it’s Cave’s face on the poster, the film aims to capture the work of a partnership whose other half is Warren Ellis. On paper, it’s not an equal association; Cave writes the lyrics and co-writes the music, plays keyboards, and sings lead vocals — he’s the clean shaven face of whichever collective he and Ellis are part at any given time (eithe...