The characters.

All reality TV shows rely heavily on characterization, from shows that have new characters each episode, like intervention, to shows with a consistent host and participants, like man vs. wild. Each of these shows have either incredibly relate-able, flawed, or charismatic personalities. Shows like Man vs. Wild have a very charismatic man named Bear Gryles showing us how to survive harsh environments. While in shows like Intervention the new characters are being rounded as the show goes on. The viewers will either sympathize with him/her, or despise him/her. This type of show doesn't rely on the characters as much as the events themselves. A third type of character on reality TV would be the relate-able character. An example would be Undercover boss. These characters tend to make the viewer sympathetic with them; and the studio will try to frame them as an "average joe." Character development is important in every type of reality TV, and different forms have different types of character development.