The existence of the table, or even just rough notes, is a tool for consistency. Yes, I’m still making a decision, whether I’m designing a table or making a judgment call in the moment. But I’m also making a decision about how decisions get made. That’s important.
By committing to a model or a process, even one with odds for irrational outcomes, I create stability in how the world behaves. Players can recognize that stability and use it as a foundation for their own choices. That’s what distinguishes a sandbox with internal logic from one driven purely by a referee's whim.
There are a lot of ways to approach how people (and factions) will react. The top 3? I'd probably say the broad categories with a whole lot of variation and mix-and match would look something like
1) What the GM thinks will guide the campaign in the direction they want
2) What the GM thinks is most fun or useful for the players
3) What the GM thinks is most likely
I don't personally fall back on #1 very often because I never really outline what I think should happen or what I want to happen. The characters drive the direction of the campaign and I simply attempt try to put interesting toys in the sandbox that they will enjoy playing with. Occasionally I adjust or make new toys based on what they seem most interested in.
I lean towards #3 most of the time. When I come up with an NPC or faction if I think they'll be important I do a very simple description, which in D&D may include alignment if it helps me think of their moral outlook. At a minimum I'll have a very simple 1 or 2 line description that I expand on when and if the NPC comes into focus. If they're likely to be central to current and imminent events then I'll dig more into their individual goals and desires. Frequently though I'm more concerned about what faction they're associated to and their relationship to their faction.
When creating NPCs I like to consider complications and complexity. Someone may be loving and a decent person but if their family is endangered they will do whatever they can to protect them even if it means doing things they normally would not. In addition a lot of NPCs will have agendas in opposition to the agendas of the characters even if it's only on very specific issues. It's just not always clear where the NPC stands.
My decisions are not based on what will be useful for the players but what is most logical for the NPC. Sometimes the NPC will be an open book, sometimes they're complicated, sometimes they have hidden agendas. Most of the time why an NPC did something will be clear eventually, but it's not always clear to begin with.