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.
I have a love-hate relationship with Greengrass. His trademark handheld style works like gangbusters in United 93 and Bloody Sunday (two films which I'm pretty sure are masterpieces), but his Bourne films only managed to awaken a hidden motion sickness in me that I was entirely unaware existed. However, the thought of Greengrass taking on science fiction, and a science fiction concept that is inherently silly no less, left me intrigued. It looked like a chance for him to try something completely new.
Part of me wants to speculate that Greengrass decided not to join the project because it would involve working under the tutelage of the notoriously dictator-ish Cameron, but who knows, really. Maybe Greengrass just really likes Treasure Island.