Depicting Robb and Jeyne’s story as it happened in the books would have meant finding a location for the Crag, actors for at least the essential members of Jeyne’s I’m into fitness fitness beer in my belly shirt(with a contract for future seasons at least in Sybell’s case) and designing Westerling heraldry for costumes. It would have been more expensive.
Also, D&D would have still had to write an outline and original dialogue for the Robb & Jeyne scenes, since the books give us the general idea and leave the rest for our imagination. What’s worse, show Robb was older than his book character, and Jeyne would have had to be as well, so even the little there was in the books – the idea that they acted out of teenage pathos – would have had to be replaced with something else.
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