There's an old addage in gaming that no plan survives contact with the players. There are pages and pages of advice saying about how you can't plan for what your PCs might do, and that anything and everything could happen at the gaming table. In general I agree with this advice. As a group it is very hard to tell what your players will do. However, what about in isolation?
Do you take the time to poke and test your PCs on an individual level? If you isolated one and provided stimulus do you know how the PC would react? Do you have an idea at least? If not, why not?
Being able to predict your players is a hard skill - and you will be wrong sometimes - but it is one that is worth developing as a GM. It lets you build specific content for people that can give them the spotlight, put them in interesting positions, and really shine certain aspects of the character. You can also cheat at this and just talk to the player. If they know their character they can tell you.
But how does it work?
Well, for example I'm going to take one of my own characters. In a friend's Star Wars: Edge of the Empire game I am playing a pantoran mechanic named Rhine. Rhine is brilliant, but also a coward with a willpower of 1. When confronted with a threat or a show of force, Rhine immediately buckles and goes with what is said. Why? because Rhine is a coward, and anyone and everyone could be trying to kill her.
Because I know this, and my GM knows this, Rhine is able to make interesting sessions for game because if an NPC gets to Rhine, she can be coerced to do about anything to keep her alive. This lets him give the bad guys a "thing" the PCs may not want them to have, show more depth of said bad guys, and a reason to give other PCs urgency because even good players take things more seriously when another PC is on the line.
Now balancing that out is another thing the GM has to do. Every adventure can't just be "Rhine has been abducted again" but at the same time, if the PCs are leaving Rhine alone, why not? Still, the fact Rhine is a coward and will react with direct self interest means the GM knows to factor that in for combat and other threats.
So, can you predict your players? And if so, what fun things has it let you pull off?