This image released by AMC shows Andrew Lincoln as Rick Grimes in a scene from the season four premiere of "The Walking Dead," airing Oct. 13 at 9 p.m. EST. (AP Photo/AMC, Gene Page)

NASHVILLE, Tenn. (AP) — Nothing's been as hard for Robert Kirkman as killing off Glenn.

Not only did he do away with a beloved character in the comic book version of "The Walking Dead," he knew he'd eventually have to face actor Steven Yeun, who plays Glenn on the hit AMC zombie apocalypse television series. Although the series departs from its source material, he knew Yeun would wonder about his fate on Season 4, which begins Sunday at 9 p.m. EDT.

"It was really strange for me writing that, knowing that Steven was going to read it," Kirkman said. "There was a concern like I didn't want Steven to read it and think I was mad at him."

Lucky for Yeun, then, that Kirkman isn't like George R.R. Martin. When fans went bonkers over the season-ending "Game of Thrones" episode "Red Wedding," Martin, author of the books the series is based on, chided fans they needed only to read his novels to know what was coming. Kirkman gives his watchers, readers — and actors — no such road map.

Kirkman and the show's creators long ago decided to veer away from the source material in key places, so Glenn's sudden passing in the pages of pivotal issue No. 100 — we're not going to tell you anything more, but rest assured it's spectacularly terrible — did not mean Yeun's days are numbered on the show. Necessarily.

"No, there's never reassurances on the show," Yeun confirmed. "Obviously, I would like to keep it going as long as possible, but it would be fun to go out that way too. ... At first when I read it I was like, 'Wow.' I thought it was brave. I thought it was terrifying. I actually loved it. I mean what a way to take a beloved character away from the readers, just snatch it away."

The Season 3 finale, which drew a cable dramatic series record 12.4 million viewers, left the comic book's fans in a titter as the epically megalomaniacal bad guy The Governor mowed down most of his followers with an automatic rifle and fled, very much alive. At that point, the show took a hard turn from the comic where the showdown with The Governor had a very final conclusion.

There are other differences that consume longtime fans of the comic. For instance, when's Rick going to lose his arm? Or will he? Why did they have to kill off Andrea, who plays a large role in the comic?

Kirkman sits in a room alone dreaming up the comic, but when he gets in a room with the show's producers and other writers, he says he's not protecting his baby.

"I sometimes am the loudest when it comes to let's change things up and let's make things different," Kirkman said. "We're all of the mind that the television show is a different animal, so while we're adapting these stories we do want to keep things fresh and new for the television audience just like it was fresh and new with the comic book audience the first time they read it, so I feel like those changes are important."

Though few details have leaked out about Season 4, Kirkman and supervising producer Scott Gimple confirm The Governor plays a role going forward and the show's main characters, led by Rick Grimes, remain in their hard-won prison safe haven with several new additions from Woodbury.

"We're doing some very new and interesting things with The Governor in Season 4, as you'll see," Kirkman said. "... The new season is about the continuing progression of these characters and their lives, so while we're still in the prison, it is a very different prison than what we've known thus far, and they have kind of built a little pocket of civilization within those fences."

In a trailer for the upcoming season, we see new faces, crops growing within the prison walls and children learning lessons — all signs of civilization. There are also walkers within the walls and plenty of trouble. In an unguarded moment, Kirkman says the character Daryl is expanding the prison population by bringing in new survivors under a special protocol to screen out the dangerous — a clear reference to future events in the comic book.

Gimple said fans of the comic will see bits and pieces like that incorporated more and more into the show. While Kirkman has always pushed for fresh storylines, Gimple acknowledges that a 16-show season means they'll likely be relying on the source material more than before.

"I use this term a lot — it's like we're remixing the comic," Gimple said.