This is an interesting question actually. In my head, “staying true to the games” initially referred to how the game operates like the other commenter said e.g.

• How different bodily needs are met. To quench my thirst, do I boil the dirty water and just take some RadAway? How much radiation does this InstaMash have? If a character in the show drinks from an irradiated lake and somehow isn’t affected by the next plot device, how “true to the game” is that? If I do that in any of the Fallout games, I’d be running into Deathclaws with only a fraction of my max HP.

• VATS. Will time be stopped or slowed down while the characters are selecting and terminating their targets? There’s a lot that can go well here especially since it’s an opportunity to inject slow-mo Hollywood-style shooting scenes, but can you imagine if they don’t put any slow-mo at all? In my opinion that would show a huge lack of understanding of the games.

• To your point, decisions. Unfortunately I think making decisions for the audience is unavoidable here unless the show becomes something like Netflix’s interactive specials. However, some good ideas might include reproducing quests similar to the ones from the games and then making decisions based on data they may have gathered from game quests. Take the Megaton Bomb quest for example. Maybe the show will force a character into deciding between blowing up a city or not at the twilight of a story arc. In the end, they decide to blow it up. Then, during the credit roll, they show that most people in the games who did the Megaton quest actually blew up the city. I don’t actually know what the real stats are, but I think it would be a good idea for the show’s characters – to a certain reasonable extent, because if we blew everything up like in my last playthrough it wouldn’t be a very good show – to follow the patterns of most decisions made by the playerbase in the games. I’d see that as an attempt to reconcile the disconnect between playing a game(lots of control) vs. watching a show(no control).