In a subtle, low-key, or hushed manner; quietly or understatedly. Usually but not always used in reference to speech or sound. I heard him state in an undertone that he wasn't surprised at the poor reception, given management's inadequate budget for the project, but I don't think anyone else heard him. In an undertone, the film upends the cliché of violence as a means of resolution that is so often the focus of other action films.