If the world that is being built is big or complex, then it probably has forces at work that the players (and their PCs) don't know about. How does the presence of those forces affect, or interact with, player proactivity, and the use of world elements as tools to achieve player goals for the game?
My current primary game is like that. The PCs are federal agents tasked with investigating and dealing with anomalous events. They have discovered at least four potent covert factions, a few particularly powerful individuals, and less organized groups with specific agendas.
Initially, the discovery of those forces comes as a surprise much like a trick or trap in a dungeon. Once initial contact is made, the players try to make sense of the force with respect to things already known in the world. The reveal may be a help or hindrance to the current PC plans, though most often it acts as a hindrance since it is an unexpected and hence they are unprepared. The PCs need to react to the reveal.
Later on, depending on the PCs relationship and how their current goals align with these outside interests, they can be help, hindrance, or not a consideration. The PCs can begin to proactively plan for their presence but still need to react when these forces engage unexpectedly.
Here's a modest example. The PCs have encountered Harry Konaklowski, a reporter for the tabloids doing effectively monster-of-the-week stories several times (he is modeled after the original Kolchak the Night Stalker TV series). The first time, he was a bit of a nuisance. His motto "The people have a right to know!" conflicted with the group's desire for a low-key investigation. Since then in no particular order, he has helped the group (he knew what they were about to face and came prepared with the material to deal with it himself), hindered the group (he was investigating a subject the PCs we're trying to keep secret at least twice), and been somewhere between almost helpful and a pain in the rear the rest of the time their paths have crossed. Much of the time Harry isn't a consideration because he hasn't heard of what they're investigating until it's complete. They do keep an eye on his column to see what he is writing about and have discussed feeding him stories to rile up other factions. A least one player thought there might be more to Harry than meets the eye and ran deeper background checks to see if he represents another faction or power group.
The best elements are ones the players may sense in the world before confrontation. They emit "ripples in space-time"; their existence affects the game world in ways the players may perceive and the actual reveal of the force resolves otherwise inconsistent features of the world.
- "A medusa! That's why there were so many polished marble pebbles along the path!"
- "She's a shape changer! That's why no one has seen her at the parties!"
- "I knew Albert and Bob hated each other. That's the only way Cheryl's actions made sense!"