I watched an episode of a show called "The Writers Room" on Sundance Channel that featured the entire writing team from Breaking Bad. They confirmed some of the things that I had suspected about their process, but I was surprised to learn that they didn't really have Walts entire arc worked out from the beginning. While I never expected them to have all the details noodled out, everything ended up fitting so well that I couldn't help but think that Vince or someone on the team had at least some idea or vision of how Walt would morph or that his end would be epic and bloody.
There were only two small things about his about his character that didn't sit well with me:
How he got from genius chemist and founder of what was to become a multi-billion dollar company to a high school chemistry teacher. A 2-3 minute flashback sequence would have sewn that up nicely. I got that there was a falling out of the founders of Grey Matter, but I would think that someone with Walt's skills could have opted for something much better than a high school chemistry teacher. Most of it was there, but I always felt there was a hole or two in that part of the story. It didn't make a huge difference, but it would have been nice to have understood that better.
The relationship with Grey Matter was a useful plot device, but it never felt as well explained as it could have been. It was almost there, but never really fully "cooked" IMHO. I understand that Gretchen and Elliot harbored at least some guilt for the falling out and then making money on the back of Walt's research, but if they were that guilty about it, why couldn't they have offered some sort of deal to him before they offered to help with the cancer? They also seemed a little too friendly for people who had started a company, broke up, and then become successful because of someone else's work... The way the writers handled it wasn't terrible, but the fact that there wasn't a little more explanation made it feel a little weak.
Apart from that, having now made it through all 62 episodes all I can say is that it has to be one of the best television shows - if not the best television show I have ever watched.
http://theweek.com/article/index/247565/8-revelations-from-the-breaking-bad-writers-room