How did it start? Good question. It started when King picked up a pencil and began to write...I think he was nine...or thereabouts. This is the best I can do and there are others who can do it better, but here's my best shot: Roland had heard about the Tower though he had never seen it. I think at one point...when he had Black Thirteen...or Black Thirteen had him...he saw the Tower, the nexus of all worlds. The worlds are in peril...who do you call when things are in peril? A gunslinger. Roland is the last gunslinger and he believes it is his quest, his mission, to save the worlds. Walter wants to possess the Tower, too and does what he can to hinder Roland's quest. Walter knows Roland, knows what he is capable of doing, had known Roland's family, knows he is the last of the line of Eld. Roland and Walter have numerous encounters, Walter doing what he can to stop Roland, Roland doing what he can to continue his quest, doing all that he can do, doing all that he wants to do, that he thinks best. I think the idea of Walter evolved from a number of ideas or characters. Or you could trace the start back further...to those who inspired King to take the journey quest down the path he took it.