|Some comments on running Ex Tempore campaigns|
The Ex Tempore world is very complex, and can be overwhelming to GM. There are so many different kinds of being in Ex, so many cultures, so many timelines.
One way of handling this is of course to run a campaign within a certain timeline or culture, limiting the phasing opportunities and the influence of other worlds. But with some planning and improvisation it is extremely fun to throw endless complexity at the players.
The complexity of Ex Tempore is deliberate: it is a multi-species society with ultratechnology. It would be just as confusing to a present-day human as New York would be to a Neolithic hunter-gatherer. Ex Tempore is a jumble of styles and sub-worlds existing in some kind of dynamic equilibrium; it is not a restful or comprehensible place. Visiting PCs should suffer culture shock, future shock, species shock and every other kind of shock the GM can imagine. They should quickly realize why so many visitors leave for the timestream.
But cruelly, when people have seen and understood the true nature of things, it is also hard to live a normal life within the timestream. Ordinary life loses its solidity when you realize that it can be changed within a Planck moment by something incomprehensible from outside or a future time traveler, and that our history just is the latest of a long series of mildly interesting histories. Many who have tried to return from Ex Tempore find themselves drawn back: it is the center of reality, even if it is such a chaotic place to live in.
One of the most confusing things happens when PCs start to jump from timeline to timeline. The GM has not only to remember what they do and how history looks currently, but also what the long-term consequences are. A change in present-day ice-cream sales might utterly transform the 45th century Andaman Empire – and if it has time travelers of its own, the changes will spread across timelines and into Ex Tempore.
It is useful as a GM to make a log of each time jump to keep track of things. Each jump leaves a timeline where other jumps or events may take place. It helps if one has a rough idea of a possible future (human) history, and then it can be adjusted accordingly.
In most cases this matters little unless the PCs go there or time travel occurs from the future, because then people from there might meet the characters. Usually it is enough to have an idea what time travel projects (or the knowledge that it is possible) are around and how they may be changed. For example, if the PCs leave from one such project for Ex Tempore, the visible effect in that timeline will be the disappearance of them and their timeship, which might either end the project or cause it to never try sending anything the Ex again.
Big things are small, small things big
The perspective of Ex Tempore is somewhat paradoxical: all of human evolution has occurred in the last two centuries, and recorded history stretches back hundreds of millennia (or more). Humans are the new kids on the block.
People from Ex Tempore might appear somewhat callous: even the worst disaster in the timeline is regrettable only in terms of how much people suffered when they died, not that they died. Everybody in the timesstream dies instantly all the time from the point of view of Aevum; the order this occurs isn’t particularly important.
It is a small, small universe
The Ex Tempore world is closed: the universe is cyclic in tempus-time, Ex Tempore itself is a closed mini-universe, and the laws of physics appear to be finite. There might be an endless aevum-future, but if everything else is finite the world will eventually start repeating itself. The limited space and resources, despite the superscience, makes recycling and keeping the environment pleasant important to the inhabitants. Beings who do not respect this might get slapped around by the mainliners.
At the same time, once an expedition leaves Ex for the timestream it is on its own. Even if it needs assistance there is nobody to turn to. Sending a message to Ex Tempore for help might in the best case lead to an expedition arriving a few seconds in the future in the next timeline – the original expedition is still lost. On the other hand, that means it is hard to stop a group that sets out to play around with history.