I see three major methods to take:
- Alternate Universe
- Study up like mad
- Narrative gaming
Explain to the players that, "Yes, I'll run the game, as I understand the setting. Consider it an alternate universe." Try to get it right. But don't worry if you get it wrong. And, if they've been asking, make certain they understand that your understanding may differ strongly from theirs.
I have used this mode in running Buffy the Vampire Slayer, Star Wars, and Star Trek. In the case of BTVS, I'd seen most of the series, but not all of season 7. In the case of Star Wars, I flat out ignored any extended universe stuff that didn't jive with WEG sources in my possession. In the case of Trek, it again ignored the extended universe; I usually run in such a variant of the Star Fleet Universe by ADB (best noted for Star Fleet Battles).
There are a couple methods for this that I've used:
- In the Past
- Exclude the following...
- The Books are Wrong...
- Over here where it's been neglected...
In the Past
In cases where the novels change what has happened since the setting book, simply state that you're setting before the novels. This works rather well for Battletech and to a lesser extent for Dragonlance.
Exclude the Following
Tell them what is and isn't canon for your variation. Make certain they agree before start. If they do, run as you would.
Works well for most games with novels. Works especially well for Star Trek TOS/TAS gaming... just ignore TNG and everything from movie 1 on.
The Books Are Wrong
I've done this with Lord of the Rings and Star Trek. Simply tell the players that the books are the commonly taught but erroneous history. They are, as written, in world artifacts - memoirs. Especially The Hobbit, which is contrived by the author to be the writings of Mr. Bilbo Baggins, Burglar.
Over Here Where It's been Neglected
Pick an area on the maps, but not covered well in the novels/movies, or covered only in novels/movies you've consumed.
I've done this with Trek (putting the explorations coreward of the known Federation) so that they don't run into Kirk and Co...
Study Up Like Mad
If you've got the time, money, and inclination, bone up and learn more. It can work. It's what happened with me and Pendragon. Some of my friends who ran it later came to me for Canon references to Mallory. (I've still never read White's Once and Future King, tho' I have read the Mabinogwion and Mallory, and seen lots of Arthuriana.)
This can be, as Randall Suggests in his answer, by use of a narrative game engine. Or it can be done with whatever engine is normal for the setting (if there is one), or even your non-narrativist game of choice.
It also has several modes...
Tell Me, Don't Ask Me
Derived from Burning Wheel forum advice...
Instead of having players ask you for information, have them tell you what their character believes. Then, set the difficulty based upon whether that answer works for you. Have them roll... if they make, it's true. If they don't, they still think it's true, but it may or may not be true, as you see fit.
If it's something that should be obvious as to what it is, and they fail, let someone else try until they get something you can work with.
Sneakily Steal from the Players
Let them discuss, and if it sounds good, adopt it and pretend you had the same idea all along. This can be excellent general GMing advice, but it also can backfire, with certain types of players intentionally twisting about based upon known weak spots...
Randall gives an excellent example in his answer to this question.
Ask the Experts I
If you have an expert in group, make use of them by flat out asking. preferably outside of session, and not in use order.
GM: What should Dr. Who do when confronted with 3 daleks modified with XM-3000 Kill-o-zap blasters?
Player-Expert: Well, in episode 7 of season....
GM: Where can I find maps of the Triangle Region of Romulan-Klingon Border?
Player-Expert: In the Triangle Sourcebook from FASA
GM: do you have one?
GM: Can I borrow it to read up for next week?
Ask the Expert II
Find a buddy who is an expert but not playing. Have them help you prepare your plot lines.
This works well, if you don't have too many surprises, and works best with the "Alternate Universe" approach also being in play.
In some cases, you may want to simply allow the group to veto/reset if you screw up. Always make it require (1) sourcing and demonstrating the error and (2) unanimous agreement of the players.
This requires you to not have too much of an ego-issue with being corrected. I've used this especially in games with shared GMing.
Some Final Advice
Which ever way you go, make it clear that any timelines are subject to change once play begins.
You're more likely to find issues with familiarity with game products than familiarity with Novels and Movies. Players are more likely to argue "You're ignoring the rules" than "you're changing the timeline," especially if they're experienced gamers. No timeline survives extended contact with PC's unaltered, but rules can be a social contract. Fortunately, adjusting to rules is easier than adjusting to unknown timelines.