How do the wizards of television keep audiences coming back for more? With so much competition it requires a mysterious alchemy to spin straw into television gold. And it’s an uber-challenge when some of the shows run a marathon of 24 episodes.

Robert King and his co-producing wife, Michelle, are show runners for the popular series “The Good Wife.”

They manage, he says, by cautiously approaching any explosive change in the plot. “We only want to go someplace if we think we have enough story to go there. And we always worry about painting ourselves into a deeper and deep corner. So we only tend to go to some place if we think we can stabilize it afterwards,” he says.

Their leading character is Alicia Florrick, an attorney played by Julianna Margulies. “It felt like Alicia’s life from the very beginning, the very moment that started, the series was about her life exploding,” says King.

“And we’ve always told ourselves the show is the education of Alicia Florrick and that can only happen with change. And so each year there’s change.”

Rob Doherty is the executive producer of CBS’ “Elementary,” the tale of a latter-day Sherlock Holmes. He says his magic wand was making Dr. Watson a woman (Lucy Liu).

“Part of the plan from the very beginning was to establish a female Watson. There was a symmetry to the plan that I liked, and the rest of the staff really liked. It also seemed the best way to break our Holmes was to say he made the mistake of falling in love once. And, again, we were excited to tell the story of a Moriarty that was devious enough to romance him and then break him down.”

What happens, though, when a favorite character decides to abandon the ship? That happened with the character of Ziva David, played by Cote de Pablo, who left “NCIS” last year to fierce fan resistance. Gary Glasberg, who oversees “NCIS,” admits it was a sucker punch. “The way that this whole season unfolded was a bit of a surprise for many of us,” he says.

“To suddenly lose a character that has been a part of the show for nine seasons is significant. I wanted it to feel incredibly different but not change the chemistry and the mix of what makes ‘NCIS’ work.”

Every series suffers its own unique problems: casting, location, scheduling, last minute rewriting. For “The Good Wife” coordinating the actors’ schedules can be torture, says Michelle King.

“Because we’re not looking to just do close-ended procedural stories, there are these arcs. And the arcs do not just involve principal cast members. They involve a guest cast. And so if you’ve cast someone, say, to be Alicia’s brother and then you need to tell the story with Alicia’s brother … and he’s not available, you’ve got a problem. And you’ll think you have someone that you’ve introduced and then suddenly they’re not there 48 hours before you start filming. So you need to be creative.”

And what do these TV gurus like to watch themselves?

Jonathan Nolan, co-producer on “Person of Interest,” says, “The thing they don’t tell you about television before you start working in it is that the second you start working in television, that’s the last time you ever watch television.

“I don’t think anyone (among these producers) probably sat down and watched an episode, certainly in three years. Maybe people talk about cable shows as much as they do because that’s the one show you have the time to kind of digest. You can watch 10 episodes of ‘Breaking Bad.’”