The books are so dense that several characters and entire storylines were left out of the show, the most glaring being the whole Zombie Catelyn Stark storyline.
Michelle Fairey as Catelyn Stark in "Game of Thrones": HBO
Warning: there will be spoilers for the books now, even though those books have been out for over ten years[...]
Catelyn Stark Archives
James Hibberd chronicled the behind-the-scenes stories of the HBO series in his upcoming book "Fire Cannot Kill a Dragon" provided the background behind the decision in Entertainment Weekly.
The origins of Lady Stoneheart is she is the resurrected form of Catelyn Stark, played by Michelle Fairley in Game of Thrones In the book, she was resurrected[...]