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Characters come and go quickly on the gruesome AMC show "The Walking Dead." Here's a look at some of the key character's we've lost.

Shane Walsh (Jon Bernthal) turned on his best friend, Rick Grimes, and lured him into the woods, apparently with plans to kill him and steal Rick's wife, Lori. Rick stabbed Shane, but he came back as a zombie and Rick's son Carl had to shoot Shane to put him down.

T-Dog (IronE Singleton) was bitten helping Carol escape zombies at the prison.

Dale Horvath (Jeffrey DeMunn) was killed by a zombie.

Lori Grimes (Sarah Wayne Callies) died during childbirth. Her son Carl apparently shot her (heard, but not seen on camera) to prevent her from becoming a zombie.

Sophia Peletier (Madison Lintz) got lost in the woods. She later turned up as a zombie locked in the barn on Hershel Greene's farm. Sheriff Rick Grimes shot her in the head to finish her off.

Amy (Emma Bell) was bitten by a zombie. Her sister Andrea had to put her down after she revived as a zombie.

Merle Dixon (Michael Rooker) is killed by "The Governor," reanimates as a zombie and is put down by his brother Daryl, who stabs him multiple times.

Andrea (Laurie Holden) was bitten by Milton, who became a zombie after being stabbed by "The Governor." She shot herself so she wouldn't become a zombie. This was heard off camera but not seen.

(CNN) -- Before heading to Comic-Con in San Diego last weekend, the cast members of "The Walking Dead" were each given a folder with talking points about the upcoming fourth season.

The folders contained information on what the actors could and couldn't say about the new episodes, which premieres October 13 on AMC. Although none of the actors could reveal the contents of the folders, it was clear that there are lots of secrets to be kept about where "The Walking Dead" will be headed when it returns.

Last season raised the stakes for the already tumultuous show by introducing a human threat to the zombie-infested world. Much of the season, which -- spoiler alert -- left several main characters dead, centered on the conflict between a man called the Governor (David Morrissey) and the group led by protagonist Rick Grimes (Andrew Lincoln).

As Season 3 came to a close, the Governor had vanished, and his community of Woodbury had joined Grimes' group in a prison. Michonne (Danai Gurira) had witnessed the death of her friend Andrea while couple Maggie Greene (Lauren Cohan) and Glenn Rhee (Steven Yeun) celebrated a very unconventional engagement.

Season 4 will air its 16 episodes in two parts, with the first eight kicking off October 13 and the subsequent episodes premiering in February. For the most part, the new season will bring fans right back into the action of last season.

"There is a little bit of a time jump, but we certainly pick up where we left off," Executive Producer and Special Effects Make-up Supervisor Greg Nicotero said. "The prison has changed a bit, and Rick has changed his position in the group. We don't really want to give it away just yet."

Nicotero confirmed that a new threat will present itself to the humans this season, and it's something that cannot be killed. That threat will be apparent by the end of the season premiere and will launch a narrative arc that he said will keep the jeopardy level high for all the episode.

Nicotero said he and showrunner Scott Gimple made the decision that they wanted the threat to be "ever-constant and ever-present" in the new season.

"So we've devised ways to put our characters in instantaneous jeopardy," Nicotero said. "That is something that was really important to us, because that is the world we live in, which is that danger could come out of anything. It was really important that we re-establish the rules of our world."

Gimple, who has written for the series for three seasons and replaced Glen Mazzara as showrunner for Season 4, said he sees this season as a "greatest hits" of the show.

"The Walking Dead" is based on a comic book series by Robert Kirkman, and although it has stayed largely true to its source material in past seasons, Gimple confirmed that Season 4 will exist in its own frame.

"It's remixing different elements from the book in different contexts," Gimple said. "Taking the emotional thrusts of those elements and giving them a different rhythm. People will see stuff from the book, and they'll know exactly where it came from, but it will be in a very different context. It has to be.

"It's a very creative, exciting thing to be doing," he added. "And you'll see stories that are totally outside the comic that you've never seen before. But the basis of those stories is the from the spirit of the stories in the comic."

The characters find themselves part of a new, larger community this season in the prison, and their roles have changed. Lincoln, whose character Rick Grimes has largely been the central point of the show, said there will be a shift in leadership as the story unfolds. This is particularly significant as Rick's wife, Lori, was one of last season's casualties, leaving Rick as the sole parent of their two children.

"He's a man that is wrestling with a modus operandi that didn't work last season," Lincoln said. "The brutality that's always going to be in him, he's trying to suppress for the sake of both of his children. He's trying to parent in an apocalypse, (and) he's trying to return to the man he once was for the sake of his children."

Lincoln said it's not been easy for his character to balance leading the survivors and being a dad.

"He's realized that the distraction and the pressure of leadership drew him away from his responsibilities as a parent," the actor said. "He focuses his energies into something completely different but still helping the people of the community, and I can't tell you what that is yet."

For Cohan and Yeun, this season gives their characters an opportunity to explore having a relationship in a (sort of) normal situation -- at least initially. Cohan said there will be "some semblance of a community and a life."

"There's some kids; I think we can say that. And there's some stability," she said. "There's hope beyond just them being in love now. As important as that was there's hope of 'Could this be OK? Are we starting again?' What happens next is anybody's business."

"You find these little moments to really enjoy life," Yeun added. "That's going to be the beauty of this season. There's a scope, and there's a diving into the world of the characters and getting to know the characters we haven't traditionally gotten to know. Seeing what they're about and seeing how they choose to live and seeing what decisions they make going forward in a horrible time. I think those little moments are going to be the key to balancing out this chaos."

As for the Governor, his whereabouts remain a mystery. Morrissey said the character has gone through some changes, however, as the events of last season weigh heavily upon him.

"From an emotional point of view, I think it's much more about what this man is carrying inside him, really, and it's about how he deals and comes to terms with the man he is," Morrissey said. "And what he's capable of. And which side he's going to fall on. That's the choice he has to make. What the writers have done for the Governor in this season is just wonderful."

A key question is whether Michonne will succeed in her desire to exact revenge on the Governor for Andrea's death.

"Michonne as a character is very invested in what happened with Andrea and I think more than any other character is driven to find the Governor, figure out what's going on with him, figure out where he is, what he may be plotting and trying to do," Executive Producer Robert Kirkman said. "And bringing him to justice. That's something her character is dealing with very early on. I can't say anything more than that."

What the writers and actors could say is that creating "The Walking Dead" is a highly collaborative and enjoyable process. Although there is always the lingering fear that a beloved character will be killed off (the producers have said that no one is safe), the cast is deeply invested in making this the best show possible.

"We all respect the fact that we're only as good as each other," Morrissey said. "We've got to look after each other."