I think the novels are a pretty good example of how this works out. You have one actual player (Harry) and several player-grade NPCs who fill the role of other players (Thomas, Murphy, Michael, etc). Generally speaking, most of the planning and heavy thinking is done by the player, with the NPCs providing occasional firepower, information gathering, and other roles (much like in the novels). Just make sure its the player driving the story, not the NPCs.