After skipping the opening-night premiere of his latest film, “Song to Song,” an experimental love-triangle melodrama set against the backdrop of the Austin, Texas, music scene, notoriously reclusive filmmaker Terrence Malick made a rare public appearance Saturday morning to discuss the film with star Michael Fassbender and fellow filmmaker Richard Linklater. The trio dove into Malick’s intensely immersive and improvisational filmmaking methods, in which no time is wasted and no opportunity for filming is missed.