In the books, you know a lot of the back story. You read the 'beefed up' version of what you see in the show. you read motivations and feelings and attitudes.
In the show, not so much. Many storylines have been eliminated or simplified.
On the other hand, GRRM rambles....a lot. He can go on for pages and pages over who's wearing what, what their standards are and what's for dinner. Some storylines plod along for the length of a book when they could have been handled in a couple of chapters. So the winnowing down that the show does, not totally a bad thing.
I also find it hard to have a mental image of any written character. The show gives me faces to put to the names.