In this episode, we somewhat controversially see a zombie use a rock to break a glass door and one is also able to scale a wire fence (while in the pilot episode, one tried to open a door). With the change of showrunners over the following seasons, the "walkers" have become more traditionally inert and are incapable of such thought-driven actions.
During filming of this episode, when Merle Dixon (Michael Rooker) is shooting from the rooftop of the department store, The S.W.A.T. squad of Atlanta were called because people thought he was a sniper. But everything was fine when they were told they were filming.
In the comic books, it's only Rick and Glenn who escape the department store. The other supporting characters like Andrea aren't introduced until they make camp. This was changed as they wanted to introduce some of the important characters earlier as the season was only running for 6 episodes.
Robert Kirkman is a fan of Michael Rooker so was thrilled to learn that he had been cast in the role of Merle.
Robert Kirkman developed a theory that walkers never attack each other, despite them all being in varying stages of decomposition and, in some cases, being freshly re-animated. He surmised that the reason why they don't turn on each other is because they all smell dead. Hence, the effectiveness Rick and his crew have experienced when they drape themselves in zombie guts.