I have a few things I'd like to discuss here.
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As a player, have you ever left a game because the GM wasn't being 'true' to the line?
Nope. If I leave a game, its because the GM was being arbitrary or just couldn't run his game in some way.
As a player, how much do you think your character knows about the setting?
That depends upon the individual PC. Most will have only the most provincial of knowledge. However, I once played Morley Dotes in a campaign based on "The Garrett Files" books by Glen Cook. That PC's nature demanded that he had to have a lot of knowledge about the setting...fortunately, I was a fan of the books so I could swing it for the most part.
(However, IMHO, Morley would have worked better as a DMPC, since he is a major conduit for information in the books.)
As a player, have you ever 'forced' out another player?
Not that I'm aware of.
As a GM, how... constrained do you feel by the fiction line?
Not at all if its fiction based on an RPG. I
never read that stuff- if it isn't in the campaign setting or its related splatbooks, its not canon to my game in that setting. You can complain about it, and maybe I'll bend a bit if I like what I hear, but as far as I'm concerned, that stuff happened in an "alternate dimension" version of the setting (or, if that hurts your feelings, my campaign is the alternate dimension).
Its a bit different if the RPG is based on a piece of fiction. As long as the stuff doesn't change basic premises of the underlying fiction- say, by having Gandalf being Sauron's older, more evil, transgendered sister in a MERP game- I'm cool with it.
And again, even then, I'm cool with "alternative dimension" justifications if they are well done but I'd probably have a problem with Evilgirl Gandalf, even in an alt. universe.
As a GM, have you ever stopped running a game because a player ruined it for you?
Nope, but I know some GMs who have.
As a GM have you had to ask a player to leave because they were ruining the enjoyment of other players?
Sort of, but it wasn't about being true to the campaign or its underlying fiction.
A player joined my HERO 1900 campaign (based in part on Space:1889, the fiction of H.G. Wells, Jules Verne, and William Gibson, and a whole bunch of other stuff) a few adventures in. He designed a PC for a campaign that, while a bit more lethal than the other PCs, was
100% thematically true to the spirit campaign.
His PC was a combat monster (as you might expect), but at one point in a combat, his PC got pinned.
I didn't make that perfectly clear, and he thought I was singling him out. Due to this misunderstanding, he left. Had he not done so, I still might have had to kick him out.
While his PC was appropriate, his overall gameplay was a bit aggressive and put off some of the other players. If I couldn't have made him alter his style a bit, or made the other players adjust to his playstyle, he would have had to go.