One of Game of Thrones' biggest departures from George R.R. Martin's books was cutting Lady Stoneheart, but its adaptation of Catelyn Stark shows why.
Lady Stoneheart was one of the biggest A Song of Ice and Fire characters missing from Game of Thrones, but the show had already revealed why Catelyn Stark couldn't become the vengeful zombie she does in George R.R. Martin's books. When Catelyn was shockingly killed at the Red Wedding in both the books and the show, that was seemingly the end of her arc. That was true of Game of Thrones, but Martin had other ideas. In the Epilogue of A Storm of Swords, the third ASOIAF book, which was adapted into Game of Thrones seasons 3 and 4, a Frey is captured by the Brotherhood without Banners, who are led not by Beric Dondarrion, but by a scarred, grey-skinned woman who cannot speak: the resurrected form of Catelyn Stark. This version of Catelyn is hell-bent on revenge on the Freys, Boltons, and Lannister, and is moving around the Riverlands, hanging those she finds.
Given Lady Stoneheart is such a monumental reveal, it was naturally expected that Game of Thrones would follow suit, with the most likely option being the end of season 4. That came and went and, despite some teases, Lady Stoneheart didn't arrive; she didn't appear in season 5 either, and eventually it became apparent she never would. Game of Thrones showrunners David Benioff and D.B. Weiss have offered several reasons for this: in the book Fire Cannot Kill A Dragon: Game of Thrones and the Official Untold Story of the Epic Series, they point to Jon Snow's upcoming resurrection, and not wanting to spoil that by having too many. They also note how great Catelyn's final scene was, and not wishing to bring actress Michelle Fairley back for a small, non-speaking part, admitting there "was never really much debate" about Lady Stoneheart's inclusion. They're understandable reasons, albeit not ones that suggest Lady Stoneheart would not have worked, but it was how Game of Thrones adapted Catelyn from the books that made the strongest case for not using Stoneheart.
Since Game of Thrones still largely followed Catelyn's arc up to the point of her death, the way it changed her character was perhaps slightly more subtle than those it killed off early or spared, but it was no less significant. In the books, Catelyn is a colder, tougher figure; she is the one, for instance, who pushed Ned into becoming King Robert's Hand and going to King's Landing, whereas the TV show had her beg him to stay. Catelyn gets a lot of unfair hate from some A Song of Ice and Fire fans because of mistakes she makes and how unsympathetic she can appear, and this is something Game of Thrones shifted: while she does make mistakes, she's also presented as a softer character who audiences are invited to sympathize with more readily.
This was particularly notable in Game of Thrones season 2, where the headstrong, politically smart Catelyn was replaced by a character who was largely defined by her motherhood and desire to get back home and be with her children, but it was clear in season 1 as well. One of the key book scenes that helped turn some against Cat - but also helps make her such a fascinating figure - comes when Jon Snow visits Bran to say goodbye. In the book, Catelyn ends this in ice cold fashion, telling Jon: "It should have been you." In Game of Thrones, while she is hostile towards him, she simply tells him that she wants him to leave, which is far more palatable. Her relationship with Jon Snow is a clear example of how the show changed Catelyn, and returns again in season 3. In a scene of pure show invention, she tells Talisa about Ned bringing Jon home:
"When my husband brought that baby home from the war, I couldn't bear to look at him. Didn't want to see those brown stranger's eyes staring at me. So I prayed to the gods 'Take him away, make him die.' He got the pox and I knew I was the worst woman who ever lived. A murderer. I'd condemned this poor, innocent child to a horrible death all because I was jealous of his mother, a woman he didn't even know! So I prayed to all Seven Gods, 'Let the boy live. Let him live and I'll love him. I'll be a mother to him. I'll beg my husband to give him a true name, to call him Stark and be done with it, to make him one of us'... And he lived. And I couldn't keep my promise. And everything that's happened since then, all this horror that's come to my family, it's all because I couldn't love a motherless child."
Again, the core idea here is to endear Catelyn more to audiences, not only to make her more sympathetic but to have her death be more painful. Even that is changed; in the books, Catelyn descends into madness from the horror around her, gouging her own cheeks which lead to the scarred visage of Lady Stoneheart, which was left out from the show. The result is still terrible, but again there's an element of things being softened by a degree, or as much as is possible amidst that much carnage and bloodshed. All of this shows that the TV show Catelyn Stark would not believably be as vengeful and cold-blooded a killer as the books, and so Lady Stoneheart would not have worked on Game of Thrones. The show never fully reckoned with the idea of death changing a person either, and since Catelyn's transformation would've been even more dramatic in the TV show, then it would've been a much harder sell, and it's arguably for the best it didn't happen, allowing show Catelyn to remain at peace, and surprises still in store for the books.
