You are James Cameron. You are producing a remake of a 1960s semi-classic about a group of scientists who miniaturize themselves to enter a human body. Who do you pick for the director's seat? Why, Paul Greengrass, director of The Bourne Supremacy and The Bourne Ultimatum, of course!
But he never signs below the dotted line. And then he unceremoniously drops your Fantastic Voyage remake to work on a "stylized adaptation of Treasure Island." Since you're James Cameron, this is the point where you go all Klaus Kinski on your bathroom, I suppose.