In his home in rural Texas, on the eve of his final tour in 2017, singer-songwriter Daniel Johnston told me that he wanted to have a big song someday, what he called “a real hit.” That smash eluded him, but the cascade of poignant, often slyly funny songs he compulsively churned out over the past four decades garnered him a cult fan base that included Kurt Cobain, Beck, Jeff Tweedy and Lana Del Rey. His lyrics chronicled universal themes like loneliness, despair and unrequited love, but did s...