In A Clash of Kings, there is a great big Jamie Lannister-sized hole. After being captured, the Kingslayer is kept in a dungeon. He appears only once in the book, when Catelyn releases him. Going practically an entire year without seeing such an important character would have been a misstep, one that Benioff and Weiss found a great way around. Instead of being kept in Riverrun, Jamie is imprisoned at Robbs camp. From a story-perspective, it makes sense. An on-the-move Kingslayer is harder for Tywin to track down. And dynamically, its wonderful. Jamie has a real Hannibal Lecter vibe in these scenes. Also, it enables them to better build up his escape attempt (something that happens offstage in the book), which of course causes the death that eventually loses Robb the Karstarks, and also the war.