Improvisation
The famous thesis that "no plan survives contact with the enemy" is especially true of a game of Solipsist (though I'd hope that the players are not the GM's enemy, even if the Shadow is). It's all very well having some great ideas for your Shadow in mind before the game, but when the Solipsists Change Reality those ideas may well go out of the window. Once the game starts the most important thing is to remain reactive, working with the players as to what the Shadow is and does. If they pick on an element of the world (for example a trapped motorist, or the spectral railway tracks, or a model in the city planning office) and deem it important then make it a thread. If they see a connection to the Shadow where you had not envisioned one, go with it.
Does this mean that there is no real value in all that preparation I just advised you to do? On the contrary the fluid nature of a Solipsist game makes it all the more important, because it is easy to lose any sense of continuity, narrative or story, when each Change can contradict what has happened before. The thematic elements you thought about before the game began give you a way to provide the players with a thread (literally perhaps) of sense that keeps the story cohesive.
How do you do that? Read on.
In Play
Remember, Shadow elements are not casually destroyed, if a Change's difficulty does not include the Shadow level then it's not going to get rid of the Shadow. This means that even though the world tends to change dramatically throughout a Solipsist story, the Shadow itself is a little more constant. Think of it like a dark whirlpool in the middle of an ocean. The ocean may be filled with waves and flotsam that get shifted all over the place, but the whirlpool remains until someone blocks it up. The same is true of the Shadow in play. Until the Solipsists deal with it an Incursion is always there in the background, exerting its influence on everything. Threads do the same, once identified they don't go away, even if the world changes dramatically, until someone makes the attempt to unravel them (or re-weave them).
As a GM this means that, if the players change something about the world without dealing with the Shadow, you should feel free to narrate it's continuing presence back into their new world.
For example
Continuing with our example, lets imagine that Grant, frustrated with the traffic snarls, road blockages and delays that are stopping him from reaching a party he's heading too, makes a Change of Reality to say that there are no cars in the world, instead all people are carried smoothly and efficiently to their destinations on moving walkways. The Change does not oppose the Shadow at this point, so it succeeds fine. What now?
Now I'd pick up where Grant's Change left off, with him being whisked of on his walkway, and then describe a mysterious snarl up ahead. It seems the walkways have been stopped to allow some sort of work to go on, installing a new transport line that cuts through the existing routes. Pedestrians are crowded together in an angry mob, held back by barriers from an empty work-site. Suddenly a train rushes past along the unfinished lines, almost unseen.
So until the Incursion is dealt with it's manifestations will always re-emerge, warping the Solipsist's Changes back towards it's own (you can see the same in the big example from the rulebook, where the roadies vanish from the Church). Make sure you don't actually contradict the direct effects of the Change though, unless they overshot and you spent a Shadow token to twist the results. Doing that certainly gives you much more of a right to mess with the effects of their Change.
Now of course the Solipsists may well want to actually get rid of the Shadow, which is generally where Threads come in. A Thread provides a way into the Incursion, and also a mechanical way to cut the Shadow into bite-sized chunks for more ready destruction. This is especially true if you use the optional Thread bonus rules below. Threads are also a perfect mechanism for continuity, because they run through the story, from scene to scene, and there is a mechanical incentive for players to follow them. Since a Thread is something vital to the Incursion, it will often be on your original planned list of elements, especially if you introduce it near the start of the story, when you will often be offering the players routes to follow, to see if they are interested.
For example
When we left off above the GM had described a train rushing past along the unfinished lines. The players may well ignore this, in which case it is a poor choice to be a thread. On the other hand if they say they want to see if they can catch this train that no one else seems to have noticed, rushing ahead to the next intersection of the line and the new rail route, then your next scene should be a Thread scene, where you let them catch up with the ghostly train, describe it, and assign some tokens to it.
Remember also that you don't have to directly follow an object to be following a Thread. If the Solipsists chose to visit the city planning office instead and try to make some sense of where the new rail line was going, that is still an opportunity to follow the Thread, just from a very different starting point.
A final suggestion about Threads : Threads can be enemies too. Not every Thread has to be a trace of the reality that's been consumed by the Shadow, it can also be a manifestation of the Shadow that needs to be dealt with. This makes the Thread's tokens rather like a monster's hit points. Until they resolve the Thread it can keep on attacking them (with Shadow scenes). If the players conceive a particular hatred of some minion of the Shadow then this might be the best way to treat it.
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