I have been rewatching the first few seasons, as I have not seen them since they aired. Whether intentional or not, I am amazed at how much foreshadowing we see in the first season alone. Even the first episode contains most of the elements we see in the later-Walt. I am not sure what the process was here, but the writers either went back to earlier episodes for images and plot devices to embed in later episodes - or they planned some of these out from the beginning.
The latter does not seem to be the case, because I recall a lot of interviews talking about how they were still feeling their way through the last two seasons as they developed. And we almost lost the show due to ratings in... Season 3?
(An aside - there was a Slate
article about watching things as they air rather than "binge-watching." It is so inane that I am surprised Slate actually ran it. NPR did a follow-up this weekend during which the host very graciously did not eviscerate the writer, which would have been easy to do. Binge-watching Breaking Bad is an excellent way to watch the show. I am picking up more details about Walt's character development than I did the first time around. The first time around was like getting a weekly dose of shock and awe that took 6 days to wear off).