How much do you and the players know from the source, and how much will you have to add?Edit
An example is the layout of the players’ starship. Think about the Millennium Falcon: you’ve seen the cockpit, the lounge, and the gun turrets which are linked by a central tube so the gunners can shout to each other. And some curving corridors. But what else is there? How many staterooms does it have? How big is the cargo hold? What does the engine-room look like – is there even an engine-room, or does maintenance depend on crawling through access tubes, or even going outside and opening hatches?
Other information is background knowledge, only important when it becomes relevant to the scenario. Is there a human overseer watching the droids emptying the rubbish bins into the bin-lorry, or are the droids unsupervised and vulnerable to nefarious reprogramming? What governmental oversight and control is there of the security services, and how likely are the secret police to break those rules?
If you are good at making stuff up ‘on the wing’ it will be good enough to make brief outlines of the way the government works, and details of the trash-collection can be left till the players ask about it. But there will be some resources that the players need to see immediately. A plan of their starship is one. It will need to be drawn in advance since it will be a fixed resource. The players need to understand better than they might know the same ship on TV. As soon as they board a ship, they will walk around it and gain an understanding of its layout that can only be replicated by showing them a plan.
If it’s not their ship, there may be places they can’t access – don’t draw them on the plan. Forget what you saw in a certain film – third-class Irish emigrant passengers on the Titanic couldn’t wander around the first-class areas and poke around amongst the valuables crated up in the hold. One of the reasons so many died was that the access between levels on the ship was through gates which were kept locked to stop them doing just that. That’s OK – the GM can map out the ship, and give the PCs a censored version of it which doesn’t show the areas they can’t explore.
The less experienced at GMing you are, the more you need to prepare in advance. If a commercial organisation has already developed the setting for a game, much of this information can be found in that material. Yes, there is a plan of the Millennium Falcon in the Star Wars game material. Or what you want might be on the Net – a lot of programmes have background material on-line, or other fans will have spent their time working things out and then posting it as information which you can freely use for non-commercial purposes. Not only can you search for specific details like the layout of a ship, but reading other people’s speculations and discussions will give you ideas for adventures.