McLeaned: Dale. Jeffrey DeMunn, who was a long-time collaborator and friend with showrunner Frank Darabont, asked to be written out when Darabont was axed. He later had second thoughts, but by then it was too late.
Reality Subtext: As explained in this interview (spoilers, obviously), Sarah Wayne Callies was informed of Lori Grimes' death while working at a refugee camp in Thailand, preoccupied with "issues about medical care and gender safety." This makes Lori's Death by Childbirth — a fate that modern medical technology has largely eliminated in the Western world, but which is still all too common in places like a Thai refugee camp — all the more ironic.
Spoiler: The details of the Blu-Ray release of the second season were released before season two was over. The blurb said that the release included Shane's last episode. Non spoiler news story here.
Throw It In: The barn's collapse in the season finale was not scripted.
Had Frank Darabont not been fired, the second-season premiere (as indicated in preview trailers, leaked information from various sources and deleted scenes in the Season 2 boxset) would have resolved the "Vatos" plotline introduced in the first-season episode of the same name. After leaving the CDC, Shane almost gets overtaken by a horde of walkers when his car breaks down, and is saved by Dale and Rick in the RV. With limited options, the group goes to the Vatos camp to warn them that things are much worse than they expected. They find the retirement home overrun (with Guillermo and the rest of the Vatos recently killed), and proceed to take down every walker in the vicinity before having to hide until the morning. The encounter with the horde of walkers on the highway is the only major remnant of the original premiere (as most of the footage was, according to AMC, "unusable") - in total, just 1/3 of the filmed material remained. The Vatos resolution also counts as an Aborted Arc.
In addition to this, there was another series of webisodes planned which would have explained the story of the soldier walker in the tank; who, as it turns out, was played by Sam Witwer. But with Frank Darabont being fired, his plan for this fell through. According to Witwer, there were dozens of other little things like this that Darabont had plans for in the future.
John Hawkes was offered the role of The Governor, but turned it down due to misgivings about taking a part on network TV.
Axel's actor revealed that Axel was originally written as a serial killer who would kidnap one of the group members after being freed from the prison.
Morgan was supposed to reappear in Season 2, but it was delayed due to Lennie James having a busy schedule.
According to variousaccounts, the deaths of Andrea/Milton in "Welcome to the Tombs" were intended to be completely different. The episode would have begun with Milton being unexpectedly shot in the stomach in the torture room by The Governor, and left to bleed out while Andrea is still tied up. There would have been more scenes of Milton trying to open the door, along with trying to choke Andrea with a chain in order to spare her from being eaten alive. The subplot was supposed to end with Milton turning, being shown taking a bite out of Andrea's chest, and Tyreese discovering her and shooting Milton. The subplot was eventually reshot for the broadcast version, and the only remnant left of the original plan is Tyreese saying to Sasha that he's going to look around in Woodbury.
George A. Romero was asked if he wanted to write for The Walking Dead, but declined, as the zombies in the show aren't used for the satire or political criticism that Romero uses them for.
Kirkman revealed that there were plans to kill off Hershel as early as Season 2.
Creator Robert Kirkman stated that in the universe of the TV show, George A. Romero's Night of the Living Dead was never created, and there was consequently never a boom of Zombie Apocalypse pop culture like in our world. This is why the word "walker" is more commonly used, as none of the characters have any knowledge of the concept of "zombies" in the modern sense of the term.
According to Talking Dead, Otis was the one who found and corralled the undead Sophia into Hershel's barn, but Shane killed Otis before Otis knew of a missing girl in the group, meaning that the whole search and Daryl's near death was all for naught.
The Video Game has a number of Dummied Out lines still hidden inside the games files;
Episode 2 includes conversations between Danny & Lee, which state that Danny used to be an insurance salesman and once practiced taxidermy on his Mother's Cat (for a "Mother's Day Gift"). There's also a nightmare sequence where Doug/Carley (or rather, Lee's Conscience) berates Lee for not saving him/her in the previous episode.
The writers were thinking of cutting Clementine before Melissa Hutchinson caught the role due to having difficulties finding an actor for the character and the fact child characters are often disliked in video games.