HBO and NBC both famously passed on the opportunity to air "The Walking Dead" — the most successful drama series in cable TV history — but it's why one of them turned down the show that's the most interesting part of the story.

"It actually did almost get made at NBC," said "TWD" universe creator Robert Kirkman on Friday's episode of "The Writers' Room" on SundanceTV. "It didn't get made because when the pilot was turned in, the famous story is one of the executives said, 'This is awesome, I really love this, [but] does it have to have zombies in it?'"

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Odd request aside, "TWD" would have been a very different series on a broadcast network, and even on HBO, which reportedly rejected the series because it was too violent (having Phil Leotardo's head smashed under the wheels of a moving SUV on "The Sopranos" — with an accompanying sound effect that is haunting to this day — apparently was not). Kirkman, an executive producer and writer on the AMC series, can laugh about such things now that the series is such a massive hit, going into its fifth season and with a spinoff series on the way.

But "TWD" is his first hit, after many failures, one of the fun facts we learned on the Jim Rash-hosted "Writers' Room":

* Kirkman launched several comic book series before "The Walking Dead," which debuted in 2003 and just hit 126 issues on April 23. All previous Kirkman comics failed to make it past issue six.

* We'd argue Norman Reedus's Daryl Dixon, one of the most beloved characters on the TV series, is also probably one of the least likely to be killed off. But Kirkman said the character's popularity does nothing to quell the actor's worry about his longevity in the zombie universe. "He's always very nervous... every time he sees me," Kirkman said. "'How am I doing in Season 5, man? Am I doing OK? Am I doing OK?'"

* Even though AMC had signed on to do the series, Kirkman said he and the writers wanted to know just how far they were willing to go with the violence and other uncomfortable plot choices inherit in telling the post-apocalyptic story. And he said when the network didn't balk at Rick Grimes (Andrew Lincoln) shooting a little girl, a zombified little girl, in the pilot episode, he knew AMC execs were truly behind the vision for "The Walking Dead."

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* Having said that, Rash asked if there was ever a scene or storyline in which Kirkman fretted the show had gone too far. "No, is the short answer," he responded, but admitted they were concerned when filming the brutal Season 3 scene with Carl witnessing his mom Lori giving birth to Judith and Lori's excruciating death. "That was a tough one. There's always those discussions of, is it OK for Carl to be present here? Is it too heartbreaking a scene? Are we going to lose people here? But we always end up going for it. We always want to be true to this world and how harsh things are."

* Though the comic books inspire the series storylines and characters — with many notable differences between the two — Kirkman said he's very careful not to let the series affect his plans for the comic book, especially since it's likely the comic book will continue well beyond the TV show. "I have often considered the fact that the television series is going to come to an end before the comic book series ends ... and because of that, I can never tell anyone involved in the show what my plans for ending the series are. Because there's the very real possibility that it could end up in the show before I can do it in the comic, which is not how I want to do things."

* Random pop culture fun fact from a "Writers' Room" onscreen graphic: The first San Diego Comic Con, in 1970, had 145 people in attendance. For the 2013 Con: 130,000 fans, and that's only because organizers had to start capping attendance because of the event's popularity and the limited space at the venue site.

* No one likes a "Walking Dead" spoiler, not even the show's writers. Kirkman said his fellow "TWD" TV writers were angry at him when he told them about the death of a major character in Issue 100 of the comic (it's a character still alive on the series). "Everyone in the room was like, 'Why did you? What?! Come on!' They were pissed off that I spoiled it."

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* About that upcoming "TWD" spinoff series: It will feature a completely different cast, be set in "a new corner of the universe," Kirkman said, and find the apocalypse survivors living in "very different circumstances" from Rick and his group.

* Kirkman on the upcoming fifth season of "The Walking Dead": "Every season we come in with new characters, building up a largely new cast and new situations in a new environment. We don't do that thing that I think successful shows would do where they go, 'OK, this formula works, never change this!' So by the time we get to Season 5, the world will have matured to a point where we're going to some really interesting places that I think are going to shock a lot of people."

"The Writers' Room" airs Fridays at 9 p.m. on SundanceTV.

"The Walking Dead" Season 5 premieres on AMC in October.