The captions are hidden because they go between video frames, in the non-displayed part of the signal. Captions can be funny and some are better than others. The captioner creates the text on a computer, synchronizes it to the video dialogue, and inserts it into the videotape. He reviews his work to eliminate slip-ups. Television game shows and movies use this procedure.
Live shows are more difficult.
The captioner works as the event occurs either from a script or by typing as he listens. News and sportscasts are examples of live events. The script captioner only displays in real time. Before the show, the captioner loads the script into a computer. The captions appear a line at a time.