One of the things I struggle with continually is the tension between wanting a living world, and wanting to be fair to the players by giving them lots of information (since I am the bottleneck on all their information about the gameworld). In particular I struggle with offscreen NPC activities. I want to make them do a lot (move around the dungeon and get behind the PCs, sow caltrops; launch coups, explore dungeons, sometimes get themselves killed offscreen, possibly turn into ghosts) but when I can't think of a way to give the players visibility into why e.g. Komar the Terrible is now a ghost, I often don't.
Remember that there are two broad categories of things that can impact how players roleplay their characters: the environment and the social landscape in which the player characters exist. The problem you described falls into the social landscape category.
In my opinion, the only way the social landscape can impact a player's character is through how you roleplay NPCs. So, during a campaign, I focus on which NPCs the characters are likely to interact with. I also consider what factors might impact the NPCs' behavior, which will then impact how I roleplay them when the PCs interact with them.
In real life, people typically only have a few people with whom they interact on a regular basis. The same is true for players in my campaigns. I assume that most of the other people the players encounter, even in a busy place like the City-State of the Invincible Overlord, are fleeting or unimportant interactions.
This means that I typically only need to roleplay a half-dozen to a dozen NPCs. Each of these NPCs will have their own social circle, but I can simplify this depending on the nature of the NPCs' relationship with the players. I typically end up with a list of maybe three dozen NPCs for whom I need to come up with personalities, plans, and motivations. The NPCs beyond the PCs' social circle are just sketches.
I also put a fair amount of work into determining how cultures work, whether they are religious, political, social, or ethnic. I believe that every character in my setting is unique, but that uniqueness happens within the context of the culture they inhabit. To be specific, I will come up with a letter-sized page worth of notes for each culture.
It is also helpful to have a good set of random tables that reflect how you think your setting operates. These tables don't need to be elaborate, but they should be detailed enough to give you a spark when the players go off the beaten path. They should also help keep you honest about what the players encounter and who they interact with. We all have our biases, so a good random table can help diversify your roleplaying enough to make that the people and the setting have their own life.
All of this benefits from being a well-rounded reader of history. The most useful information for RPGs is the stuff that describes how people lived their lives on a day-to-day basis. The grand sweep of history is not as useful. However, you do want to have a basic understanding, because as I said about culture, every person who ever lived is unique, but within the context of their times.
With all of this in mind, you will be prepared when the players decide to go left instead of right. You will be able to extrapolate a consistent and believable way of roleplaying the people they encounter.
I apologize if this is a bit rough. I hope it is helpful!
This may be useful, I turned my World Outside the dungeon chapter in my RPG into a free PDF that folks can download. I don't go into as much detail on handling NPCs as I was focusing more on managing the entire campaign.
Could "Somehow, Palpatine has returned" be okay if there actually were a good explanation that the GM doesn't go out of their way to reveal?)
To answer the question about Palpatine's return what impact does his return have how you would roleplay the NPCs the PCs will interact with?
Maybe a former imperial officer who winds up running a local bar after getting mustered after the Battle of Endor is inspired to their part to ensure peace and order in the galaxy once again under the Empire's aegis. Then starts recruiting imperial sympathizers to ready for the day the Empire returns. Her changes in behaviors starts raising red flags to the observant PCs and before they know it they are investigating an imperial cell on their planet.
Or maybe it inspires her to finally commit to the New Republic and she uses her connections with the PCs to do more for the Republic and give them a list of all the hardcore remaining Imperial sympathizers she knows of.
Is starts by asking OK given the current situation of my campaign what happens to my NPCs if Palpatine returns?
