George R.R. Martin has provided a rough outline for what happens to certain characters/houses throughout the rest of his books, even for things that aren't published yet.
D and D take those story beats, completely strip out what led up to them (or what will lead up to them), and write it all as a television drama. Television dramas involving 200-300 named characters aren't typically rich with character detail, so I'm amazed they've done it so well.
This is why we have characters like Olly, Ros, Karl Tanner, etc. They exist to fill in the gaps of what the D&D weren't able to translate from the books.
Ros is everyone's favorite whore, so she lets us know
-That Tyrion loves whores
-That Theon is a bit of a bad boy
-That Littlefinger isn't to be trusted
Karl Tanner is everyone's favorite fookin' legend of Gin Alley, so he lets us know
-An evil mutineer kills Jeor Mormont and Craster
-The wildlings could torture him and provide the willdlings with information about the NW's defenses.
-Jon Snow is true to his brothers.
Olly is everyone's favorite wildling orphan, so he lets us know
-The wildlings kill innocent people.
-The night's watch hates the wildlings.
-The night's watch are going to hate Jon for siding with the wildlings.
Littlefinger and Varys, although in the books, exist similarly to give the viewer information about backstabbings and dealings around Westeros. These two characters apparently know everything. Littlefinger in particular can travel thousands of miles in a matter of a few days to locations that take every other character months. Melisandre apparently has this power as well, since she traveled from Dragonstone to the Riverlands and back again in no time flat, and also recently from Castle Black to just outside Winterfell and back again.
Beyond that, the show is a series of
-scattered "purgings", such as the red wedding, Stannis' slaughter, the purificaition of King's Landing, the murder of the Stark men at KL, the murder of the Ironborn at Moat Caitlin, the masters in Astapor, etc.
-Buddy romps around the countryside to remind you of what's going on in the story, and how characters feel about certain things, such as Brienne/Pod, Jorah/Tyrion, Arya/Hot Pie/Gendry, and coming next season, Jorah and Daario.
-Armies or large groups of people on a quest for power, such as Stannis, Robb, Dany, etc. So far only one conquering group has succeeded.
-Characters who exist in locations either against their will (Sansa in King's Landing) or in great distress (Bran at Craster's keep, Brienne at the Dreadfort), or perhaps characters who are there to provide a little evil for the good guys (Roose in Robb's camp) or good for the bad guys (Tyrion in King's Landing)
-Finally, there are power trades/negotiations, such as those between Littlefinger/Margaery, Renly/Catelyn, Robb/Greatjon Umber, etc.
-And of course, the small council. They don't actually do anything at these meetings except for issue orders of execution and remind people what is happening in Westeros, similar to Varys and Littlefinger.
Then there's Satan, the character who has now personally murdered more men than every other character in the show combined. Ramsay Bolton.