So he began to write his first draft in order to get the plot down. When he felt he had gotten the plot fleshed out really well, then he'd go back through each scene and insert setting details, then go back once more and add in character details to make his characters more real and make sure they were maturing and growing at the right pace.
This advice has served me well over the years. Usually I use it when I am editing after I feel I've finally gotten the story "right." It really helps me to be sure that each scene has character, plot, and setting, and that I haven't left anything out.