One rule of thumb you very often get to see about planning and running a game and also writing stories, is that the characters should be “acting instead of just reacting”. But I think except for special cases in which the PCs are trying to establish a domain or something like that, this is not really a good way to describe the issue.
In virtually all cases, every story, both fiction and academic history, is about dealing with an extraordinary situation and returning things back to normal. What is considered “normal” depends entirely on the perspective of the people who are in the center of story. If you have a society in which group A keeps group B in slavery, and has done so for generations, the state of slavery would be the “normal” state of things for people who side with group A. But for those who are siding with group B, the situation would still be extraordinary. It just has been that way for 200 years. But when someone does start getting active in any way, to end the state of slavery, from his perspective it will be all about turning things back to what he considers normal. It does not matter if the extraordinary situation began 5 minutes ago or has been going on for centuries. Any character who feels he has to change things or stop an ongoing crisis does so because someone else, at some point, upset the normal state of things.
And to turn things back to the way they should be is always a form of Reaction. If the antagonist has any goal more complex than getting rich, he will see his actions as a reaction to an undesirable state of things as well. There are very few cases in which any character, both protagonist and antagonist, does not believe he is reacting to a disturbance started by someone else before. Coming up with narratves in which the characters are pro-active is still good advice, but it would be a mistake to try too hard to not have the protagonists be reacting. What you should be avoiding are Reflexes. Don’t put the characters into situation where there is really only one obvious choice what they could be doing in response.
I’ve finally got around to playing The Witcher 2 these lasts days, and there’s a number of moments that I would just love to encounter in a pen and paper game. Unfortunately, because of the limitations of the medium of video games, it still comes down to chosing pre-planned path A or path B. But from a writing perspective, it’s still interesting. In one scene you know a villain is going to attack one of your friends, and so you race back to the town as fast as you can to get there before it’s too late. (So far, a typical case of a reflex. It’s obvious you can’t wander off hunting monsters or exploring ruins at this moment of the story.) But when you get to the town, there’s fire and corpses everywhere and bands of murdering thugs chasing more victims down the streets. Now if that scene would happen in a pen and paper game, what would you do? What would be your reaction? Ignore the chaos and continue to run to the house where your friend is staying? Start beating up the thugs in sight? Or go to the quarter of the town where the minority they are hunting has their homes to get as many of them as possible to safety?
I think that’s the kind of situation that so far I’ve always failed to provide in the campaigns and adventures I’ve been running for the last 15 years. Instead of “You find a map on the corpse of the bandit leader you just killed. Do you want to check out the marked location?”, I think it would be way more fun and exiting to have more moments that are “Shit is happening! What are you going to do?!”
Putting characters into situations where they have to react is not bad. The key to creating good stories is not to avoid the characters reacting, but to put them into situations where they have to decide for themselves how they want to react to them. What you should be avoiding is situations in which the characters reflexively do the obvious thing without any real alternatives.