Whether the word was “cherishable” or “perishable” was a little hard to make out, on account of Frank always wearing that big papier-mâché head, but anyway he saw something in our protagonist, a budding young musical mollycoddle (Domhnall Gleeson), and decided to hire him. As is the newbie’s privilege to explain, Frank, this inscrutable pied piper of art-rock obscurity, has his ways. Is it sort of a spoiler to declare that the title character in director Lenny Abrahamson’s dark comedy, whose face we almost never see, is played by Michael Fassbender? Maybe, but it’d be worse to go through the movie wondering, “Who is that?” Better to know and to anticipate the long-delayed reveal; contrary to some opinion, the most stirring Fassbender stuff, anatomically, is in his eyes. While we wait — holing up in the woods to make a record, seeing Maggie Gyllenhaal surpass typecasting as a severe, theramin-wielding bandmate, and trying not to choke during an important South by Southwest show — it’s all in the voice and the posture, the way the head weighs him down. Though packaged for relative ease of consumption, Frank does manage to investigate the frontier between indie quirk and mental illness, the volatile combination of tortured soul and personality cult. Mystifying, demystifying, it seems a little at odds with itself, but then so does Frank.