It's part of the disparity in information presented to the players. Unless you (the GM) state that the car about to slam into them is yellow, that car could be any number of colors, and each player is likely to imagine a different color in their head.
A blacked-out SUV definitely brings a different image to mind than a tricked-out low rider with custom rims.
I generally don't see a lot of focus on how to improvise well, there's always an emphasis on the GM doing the heavy lifting for plot.
First, players who can drive plot and take action without having to purely react to everything are amazing and awesome and you should jealously prevent these players with threats of bodily harm from joining games with schedule conflicts with your own. I'm not talking the dingbats that run off doing frivolous shit that only their character cares about, I'm talking the players that hear that a rival mob boss recently got out of prison and does something about it.
They don't wait for that guy to attack, they decide that they want to scout this situation out now.
The GM isn't Atlas, carrying the entire plot is a recipe for burnout and frustration.
Second, almost everybody plans too specifically. Information that is required for the story to move forward needs to be accessible to the group- and people frequently hide that behind hidden doors, passwords, or in other crafty locations that players never consider to look.
Again, disparity of available information. To the GM things seem obvious. To my players "Out in the harbor" doesn't imply that SOMEONE IS ON A FUCKING BOAT IN THE HARBOR.
Tip: In modern games, especially TL 8 (and for part of TL 7), cellphones and computers will be the go-to source of info being held by an NPC if that NPC doesn't want to share over coffee. I should probably write a post about how much of a
Third, trying to plan for PC actions is near impossible, although part of that is familiarity with your group. Some groups are far more chaotic than others- the Agency 17 boys never made a severe attempt to truly go off the rails. Other groups actively run away from plot.
If your players run away from plot, I suggest getting new players or doing a much better job indicating what your game is about so people buy into the premise.
We're talking about tabletop roleplaying, not writing a book. The GM gets to react to what the players do, just like the players react to what the GM throws at them. Some of the best moments ever are when a player thinks outside the box and comes up with a solution to a problem that is both cool and successful.
Fourth, as a GM you really should be thinking about cause and effect and logic. I've blogged about this previously.
Really improvising well draws on knowing appropriate consequences for the players actions, knowing how the setting, situation and NPCs should react to various occurrences, and making sure that it's all consistent with one another.
So the players have caught you with your pants down. You expected them to accept a deal with this certain duke/capo/politician/government and they decided to fly dual-middle fingers to everyone and go for plan B (#15).
So... uh... what do?
- Relax. Breathe. Sure, things have not gone as planned, but you got this. Worst case scenario, feel free to end the session and state that you need time to plan around what happened. Your players will feel awesome for doing that to you If you only need 15 minutes, go on a 'bathroom break'.
- Do not worry about the plans you had. They are smoke now.
- Focus on the now. You only have to get to the end of the session. Red tape and bureaucracy are your friends here. There's tons of crafty ways to delay player actions to drag things out when you need to slow down the pace of a game. See #57. Be devious.
- Use logic. People use internally consistent logic to make decisions, even if they can't explain the logic or don't understand the logic. NPCs are the same.
- Let the consequences happen. If your players ask for trouble- give it to them! So what if that powerful duke/mayor is angry at the players now? Let it happen! Your players have just livened things up and asked for a major challenge! Roll with it!
- Decide later how things shake out on the bigger picture.
It gets far easier with time and practice. Plan less, think more, react better.