Imagine a common movie scene: a hero confronts a villain. Captioning such a moment would at first glance seem as basic as transcribing the dialogue. But consider the choices involved: How do you convey the sarcasm in a comeback? Do you include a henchman's muttering in the background? Does the villain emit a scream, a grunt, or a howl as he goes down? And how do you note a gunshot without spoiling the scene?

These are the choices closed captioners face every day. Captioners must decide whether and how to describe background noises, accents, laughter, musical cues,...