For her feature film debut, the British artist Clio Barnard has staged an experimental biographical documentary on playwright Andrea Dunbar, whose rough, short life ended with a brain hemorrhage in 1990. Emerging all too briefly from a grim working-class West Yorkshire pit of domestic violence and desperate self-medication, Dunbar left a many-tiered legacy, including her gritty autobiographical dramaturgy and a handful of tragically haunted children. Along with scenes from Dunbar’s play, “The Arbor,” performed on an open lawn in her old neighborhood for an audience of local onlookers, Barnard makes enterprising use of intimate audio interviews with the playwright’s family, getting actors to lip-synch the recordings on camera with mesmerizing precision. The technique isn’t new — it’s sort of like Nick Park’s “Creature Comforts” series, except with actual humans instead of animated zoo animals, and so a lot less cute — but in Barnard’s hands it is especially powerful, at once inherently protective and singularly revealing. This is an eerie and mysterious film, more than merely a portrait of the artist as a young casualty of disadvantage.