You're fed up. You're considered the world's most dangerous assassin. A lot of people want to take you down. And you still have to get your job done. Mission Critical is the latest in Mark Greaney's Gray Man series. This one involves state actors, espionage, a secret prisoner and a race against time. Mark Greaney sits down to discuss his career, his Gray Man series and espionage with Stratfor's Fred Burton.