Okay, interesting question:
When does "Keeping a story on track" become "Blocking someone else's creative input"?
The answer depends on who you ask. I can only give you my answer.
"When there was creative input that got blocked".
It might sound almost like weaselling out of an answer, but it's not, just consider it. If there was creative input that got blocked for a reason other than "it's crossing the boundaries we set together before the game", it's blocking the creative input of another player.
Now, said boundaries might involve different stuff. Genre inappropriate stuff (it might be advantageous to use a gun in a kung-fu fight, but the characters in the movies slug it out empty-handed) can be banned. Something that crosses OOC boundaries of what another player would tolerate, or that would make the game less fun for him or her, like the OFFs we have at this site.
Or the setting might be a boundary, like you can't bring the dead back in Exalted. You can raise them as spectres and zombies, though.
A system might be such a boundary, especially if it reflects the setting. The good ones never become an issue, IME, but I've also seen more than my fair share of poor ones.
Where it gets thorny is when "the story the GM wants to tell" is acting as the guideline. However, as long as everybody agreed to follow said story, it's more than a valid one, and in that case, you take whatever actions are necessary to "keep it on track". Generally not my favourite, but not blocking anything if the players want to follow it.
If the players had discussed said story beforehand, and maybe added your own touch to it, but the are expected to follow it? That's a bit more of a "shared" story, although one player is probably still playing the primary role. But again, if that's what you agreed to, it's still under the same guideline.
If the players have a starting situation and a premise, and are expected to bring their own story by their actions in the game? "Keeping the game on track" becomes redundant. It might well be on track even if all the characters are heading to their glorious or not-so-gloriuos and tragic, "dirty dozen dancing on the tomb of Tarantino" style deaths! In my book, that makes for one less thing the GM should keep track of, and generally, that's the level of freedom I strive for. Played such games, and they were lots of fun.
And yes, the different approaches lead to different amounts of pre-planning, as we discussed in the other thread.
Does that answer your question?
Lets take a fairly straight forward group RP of the sort that there are probably dozens being played on this site alone right now: A group of adventurers set off to find the legendary Macguffin. I'm pretty sure this has already been discussed elsewhere, but the fact that I can't remember what the answer was probably means there was no satisfactory answer...
So, our adventurers leave the tavern and the party leader (played by GM) says "Right guys and gals, the map says the Macguffin is in a cave in that mountain up there."
To which the bulk of the party turn around and say "Okay, you have fun, we'll be in this tavern having sex..."
Yeah, I know, it's unlikely to happen, because if you wanted a sex-romp, why did you sign up for a Macguffin hunt? Extreme example to make the point.
Wait, are the players still willing to hunt the Macguffin after they have their "farewell to the ladies" (or whatever floats their boats)?
If yes, what would change in your Macguffin-hunting, I'd just expect you to say "nice idea, is likely some of us might die on the way, so let's fuck before!"
If not, yes, there's probably a reason to it. If that's the majority of the party, pause the IC right there and ask them OOC. That's why we have an OOC thread.
If it's just a couple players and the rest want to get on with the plot,open them a new thread, tell them to screw there, and go on with your quest. Just make it clear OOC that you're not waiting for them.
So, is the GM justified in that circumstance in saying: "Fine, see you when I get back... Oh, by the way, a demon who can only be stopped by the Macguffin has just tunnelled his way from the underworld and blown up the tavern... Now get your acts together and lets go find the damn thing!"
Is that kind of thing part of the genre you're emulating? I'm serious there, they might be willing to play "reluctant heroes", and expecting this reaction!
If they just wanted a "farewell romp", seems like you have a different idea of how E.-style "MacGuffin-hunting" plots are supposed to go. Go OOC and resolve it, it's neither a rules problem, nor an IC problem! Different understanding of the genre often leads to different expectations. And when expectations clash, that's 90% of what causes problems with games.
Or would a better approach be to say "Well okay, but I thought you guys wanted a Macguffin hunt. If you don't then fine, we can always do that some other time..." And join them in the tavern!
Generally, my approach would be closer to that, as you can see above
. As pointed above, I find OOC issues are solved best OOC, and IC issues are solved best IC. Mistaking them is often a recipe for disaster, boredom and other inglorious, game-ending fates