**Trailblazers of a Lost Art** Little wonder James Cameron and Joss Whelon movies are the biggest box-office earners. They are masters of cinematic rhetoric. The unfolding dramatic situations and controlled dialogue are meticulously contrived. Cameron could probably have potted more if it wasn't for his earnest, simplistic messages (rich bad; nature good). All three movies (_Titanic_, _Avatar_, _Avengers_) plot along comfortably then suddenly spike spectacularly. But no one has ever laid on the cinematic charm and cajolery like Stephen Spielberg. He was by far the craftiest manipulator o...