"First looking for it, I had the hardest time finding where Walter lived. Then in a conversation at dinner once, someone was talking about depression, and I went, ‘Woah, that’s where it is.’ He was a depressed man from missed opportunities. Over the years, his entire adult life, he would just gloss over, keep pushing it down.
In broad strokes, depression manifests itself in two main ways. You explode, and blame everyone for your misfortune. Or you implode, and that’s what happened to Walter White. He just became invisible to himself. Once I caught onto that, that informed everything.
Then I went to Vince Gilligan and said … ‘He should be heavier … He should be unremarkable in every way. He should have a mustache that makes people go, “Either grow it or shave it!”’ I said I wanted it to look like an impotent mustache.
So once I found that, everything changed. As the show went on, what happened to Walter White is he took a stick of dynamite to that calloused-over core of emotion and blew it up. That’s why he became so careless and impulsive. He changed from a methodical scientist into this man who took over his life.”
When Zoller Seitz observed that Tony’s struggle with depression was also central to “The Sopranos,” Cranston quipped, “That’s probably where I got it from.” (via)