As an example, let's have a look at a proposed GHOSTBUSTERS game.
In the game I pitch to my group, they will play the part of the London franchise of Ghostbusters. I'm probably picturing something that follows the basic arc of the movie: an escalation of supernatural events in the city that build to a big crisis event that could destroy the city -- and the world! As a subplot, again straight from the movie, we see the franchise struggling to exist financially and gain the respect it needs from the city authorities.
Based on that basic outline, the Story Arc of the series might look something like this:
Although this is a very structured outline, notice that it does not make many demands of the players -- it assumes that two conflicts will be created, and that the players will grapple with them throughout the series. They players are not required to react in a particular way, or constrained in their activities. The GM can adjust his perspective on the series on the fly, if the PCs manage to perform particularly well and keep in the city's good graces, for example, perhaps changing the conflict to "the London Ghostbusters are so tight with the City they're heavily relied-upon, and under constant pressure to perform". This could have the same end result of a blow-up and reconciliation, like in the movie.
Structure is not intended to limit player behaviour, only to make sure that each episode has plenty of content. The best way to do this is by creating multiple layers of story, with the most important layer being focused on the players.
Knowing this, I would have a conversation early on with my players about what their characters were like, what was important to them, that sort of thing -- trying to find places where there are conflicts to build story with.
In our example, let's say that my players come up with the following scenarios, roughly based on characters that are close to the characters in the original movie:
- One player, taking the part of PETER, wants his character to be involved with a romance storyline. That's straight-forward enough.
- EGON's player is more interested in the weird science side of Ghostbusting, and he requests a story that focuses on the ethical implications of Ghostbusting.
- RAY is the team's occult expert, and his player says he wants to have a storyline that focuses on his character having some kind of dark and mysterious past.
- WINSTON is a blue collar character, and that player requests a story that includes his family in some way.
Again, each story idea for the four characters -- which each have a "spotlight" episode that forms the body of the series -- is only a starting point, not a railroad. The Winston episode assumes only that he will interact with his family, while the Ray episode has him discovering a dark secret -- not assuming how he will react to it. There is no need to micromanage things further than this; throwing the characters into a situation that requires they participate in a conflict is all that's required to make a solid session.
Painting in a little more story in each episode, using the overall arc of the franchise battling the supernatural Big Bad and the city's bureaucrats gives just enough depth to create the illusion of a highly constructed story.