During an interview, Robert Kirkman claimed Norman Reedus (Daryl) originally auditioned for the part of Merle Dixon. He was rejected, but his audition so intrigued the show-runners that they created the part of Daryl just for him.
The word "zombie" is never used. The zombies are either referred to as "Walkers", "Geeks", "Skanks", and to the CDC, "Test Subjects". In season 2 the terms "Roamers", "Lurkers", and to Hershel, "People", and "Lame Brains" are added. In season 3 "Biters" was introduced to the lexicon.
In an online interview, it was revealed that Frank Darabont envisioned having several self-contained episodes that would not continue the main story. One such episode would have been the season two premiere. It would have involved Sam Witwer (who Darabont used in The Mist (2007) and played the tank zombie in the series premiere) as a solider in a Black Hawk Down (2001) style Atlanta. Witwer would have encountered several of the cast before being bit and locking himself in the tank.
The trivia items below may give away important plot points.
The description that Darryl gives of Merle's drug dealer (who most likely sold him blue sky meth considering he had it in his bag earlier in the series) sounds an awful lot like Jesse Pinkman from Breaking bad. The drug dealer has some sort of child in his life (Brock), in described physically as small and skinny, and says the word "bitch"
The place where the Governor, Martinez, and Shumpert were held up in season 4 episode 6 teaser was the same place where the Governor and his army had wiped out the coast guard in his first appearance season 3 episode 3
All of the seven main characters to die as of the end of the fourth season (Dale, Merle, Governor, Lori, Andrea, Shane, Hershel) have all been brought down mercilessly by another character, whether it's before or after reanimation.
When Bob, Tyreese, Daryl, and Michonne hear the Terminus broadcast message from the radio, it was eight days before Bob would make his way there and nine days when Daryl and Michonne would. Tyreese has not made it there yet