The “plot” of an event is much more than the story run by the directors and supporting cast. There are stories that emerge from character personalities playing off one another. You cannot write these stories, by their nature they emerge spontaneously from gameplay. You can, however, write scenarios which reward cooperation, competition, creativity, ingenuity, and initiative. This is the perfect backdrop for players to showcase their character concepts.
Here is one method to encourage emergent plot: create a resource that players will compete over to accomplish their goals.
Eyes on the Prize
Allow the players to choose their own goals. They should have a number of options. Participation in the plotline gives them clear steps they can take towards accomplishing those goals.
The Macguffin
The plot should involve a resource that many people could use to accomplish their goals. This could be an item of power, access to a learned sage, command of a military force, the favor of a foreign ambassador, or ownership of a famous shield. Maybe if they succeed at the challenge, the players may choose their reward, negotiating it with an NPC. In any case, everybody wants it!
Competition
Players will be competing along a common axis. If they are competing over a scarce resource, working towards one goal may prevent other players from achieving their goals. This creates inter-player tension, strengthening the importance of in-game friendships and rivalries.
Examples
- A local lord is concerned with the goblin population feeding on his crops. He has offered a reward for whoever brings him the most goblin ears. The champion goblin slayer will be given a forge or laboratory of his choice.
- There is a shrine in the woods which is a focal point for the balance between order and chaos. When meditating there, you can sacrifice an elemental component or an elemental gemstone (a treasure that can be found throughout the adventure weekend) to slightly shift the balance. This changes the power level of certain creatures over the weekend and influences the behaviors of certain NPCs.
- A number of merchants come to town, each one representing a different cause or faction. Some are friends and some are rivals. They are taking donations for their cause, and specifically looking for certain items owned by other merchants, which can be bought and sold by players. Whichever faction has made the most money by the end of the weekend will become more powerful within the season plot. Factions may promise boons and favors in reward for help with their collections.
Nerology 
#1 by Michiko on May 7, 2010 - 1:35 am
Emergent Plot is often evident when players from multiple areas get together. As stated in the Nerology, local “flavor” does happen through the realms of NERO. When groups of local renown meet at an event, there can be trades of items (both IG and OOG) and information, as well as other stories and shared opportunities at the event.
As the Exotic Safari, my group had a number of functions at various events. At ARGO, PRO, DC, CT, NH and many other chapters, we traded and created magic items for many players who otherwise would have not been able to make those items. When we met another group that was rumored to have conflicts or that was actively against others from our home land, we often spent time discussing relevant issues IG and making pacts to fix them IG.
All of this is part of the game as well. Without Player to Player Interaction, NERO is infinitely more difficult to play and enjoy.
Going to the National event in IL reminded me that part of NERO is meeting other players, part is playing with other groups, part is seeing other areas, and the last part is making your own fun. Somewhat unplanned, I spent 6 hours selling magic items to people, and that was a prime example of emergent plot between a character and other character(s).
A great example of emergent plot happened at the National event. As my character, I invited two unknown people on an adventure with our group, myself and two friends. We did some introductions and found out that we knew each other by name, but not face.
This emergent plot was not planned, nor could it have been, by “Plot.” We diligently aided each other in the adventure and a good time was had by all.
As a PC, I try to remain open to multiple types of entertainment and adventures. I don’t feel that it is solely upto “Plot” to make everything perfect because it is also highly reliant on the attitude of the players as well, both NPC and PC.