Movie stardom is a fairly easy thing to define, and since he’s been one for long enough to know what it takes, George Clooney is reasonably placed to comment on the industry’s definitive onscreen icon. The best actors aren’t always movie stars, and movie stars aren’t always the best actors, but what separates a mere thespian from a bankable name are the things you can’t teach: appeal to the widest possible audience, innate charisma, and the inde…