[RPG] How to respond when players stray from the prepared material

If you are fine with an online solution, Obsidian Portal is one I could recommend. Some people have had bad experiences with the interface being clunky, but I think the site looks quite fresh and responsive (I think they have had a design overhaul in late 2013).

Some key selling points:

• Wiki pages, which you can interlink quite easily, allowing you to keep track of things that are related. Wiki pages also support public and GM-only information to be saved.
• Adventure Log, which is where you can post the notes of your sessions and write down the overall progression. It allows you to Wiki-link, so you can interweave your Adventure Log with your Wiki pages.
• Create characters, but not just PCs, NPCs as well. Again, you can have GM-only information here, allowing you to create major plot NPCs in advance and 'unlock' them when the time comes.

For organizing an open world, I would personally create some Wiki pages to comprise a town. Using links, you can connect the page about the town in general to the big landmarks in that town, like a church, the blacksmith and stables. They would in turn have their own Wiki page, which could state in which town it resides, and which NPCs live or work there. Following the NPC link, you'd end up on a biography and stat block page of the NPCs, where the biography could link back to the town page, and the NPCs building page.

Using this approach, and combining it with Vereos' answer, you can create an incredibly potent spider web of pages, while still being able to easily navigate from one point to another.

• Each category from Vereos' answer could be its own Wiki page, which grows gradually as your players progress.
• Each item on such category page would simply be that, an item in a list. You'd use Wiki-links to jump to that item's own page, which would give you the details. This keeps the category pages clean and simple.

I use graph theory. All you need to do is to have NPCs (and/or places) as nodes and plots as arcs. You can even use something like GraphViz to visualise the graph you created. In general, the more complex the graph, the more potentially complex the plot.

Each link could have a cost associated with it that depends on how hard whatever the arc represent is to find out from the PCs's point of view. Or rather what you think is the difficulty in getting that particular thread. This can be highly subjective and difficult to do as all the information you pass to the players has the same colour! They do not know what is really important and what is not. Then you can do some "shortest path" calculations to see how reachable an end node is from any other nodes.

A limitation of this method is that it is very static -- unless you have a nice little script that you can update for all nodes/arcs as your PCs will do things that the NPCs will need to react to.