1. Steve Urkel - Family Matters
Few people remember that Family Members was originally intended to be a spin-off for the show Perfect Strangers, focusing on Carl and Harriette Winslow. Until in one episode, Carl decided to find the most nonthreatening boy in the world, and have him take his daughter Laura to the dance. The result? Steve Urkel. His effect on the audience was immediate; by the end of the episode, they were chanting, "URKEL! URKEL!" whenever he would show up. They immediately created a bigger role for Urkel, making him the next door neighbor of the Winslow family. As the show continued, there was a gradual shift away from the Winslows to the antics of super nerd Urkel. You can tell from the promotional material exactly where the show's priorities were -- after a few seasons, Urkel is front and center, with the Winslow family crammed into the back of the shot. What started as a family show in the vein of Family Ties or The Cosby Show quickly became the "Look at What Crazy Thing Urkel Did This Week" show. And to be perfectly honest, the less said about Urkel's alter ego Stefan, the better. Regardless, this is the ultimate example of a minor character totally taking over. In fact, most people don't even remember that this show was called Family Matters -- they just think of it as The Steve Urkel Show.