I generally go in pretty light into game sessions. I do a ton of background work first on the “world the characters might interact with” – so that I have a feel to the area, people, conflicts, etc. Who is who, and so on.
But it comes the actual session, my prep is something of a story board. I think of key scenes, key decisions to give the players, and maybe come up with a few stat blocks or random activities. If they are going to explore a ruin, I sketch it up. I try to throw down a few sample names, so that NPCs can have more personality than Bandit 3. I avoid references deep Glorantha lore and tend to use titles (White Moon or Horned Snake or Earth Lady) rather than the name of obscure deities (unless the point is to use the obscure name to obscure the identity).
But that’s about it. Keep it loose, and let the players really drive what happens and interpretation of what is going on. The stat blocks are there in case I need to have a contest and are often there just as a statement of difficulty. POW 25 (this should be difficult), POW 14 (this should be pretty easy). That kind of thing.
Again that is just how I approach running game sessions – be it RuneQuest, Cthulhu, Ringworld, Pendragon, whatever.
Another thing. If I am running a game set in Pavis, I really don’t need to figure out the secrets of the EWF or the God Learners – all I need for my adventures is to know that there are secrets, and MAYBE the players will want to investigate them. Don’t worry about defining this stuff until the players decide that they really want their characters to go down that rabbit hole.
Get the big contours of things right, define the things the adventurers NEED to interact with (are they going to Kakstan’s Art Gallery? Then we probably need to define what that place is!), but other things keep loose until the players decide to interact with it.
Now you might want to create some secrets that help you the gamemaster interact with things. Is there a secretive cabal behind the various Chaos cults in the Rubble? Cool! Is Prince Kallyr pushing headlong towards her own doom? Cool! Is the Lunar Empire going to make a big push towards reconquering Sartar? Cool! All of those are things that facilitate you running a dynamic setting where not everything going on is a reaction to player action. But again, you can keep those loose as well.