If you look back through the archives, I keep claiming over and over and over that "Your Game Is Not Literature." I use the word 'literature' to include novels and short stories that most people wouldn't consider literature. Different rules apply to fiction than apply to games.
Much as we want to believe otherwise, gaming generally isn't cooperative storytelling - it's more like guided group therapy where one leader guides us through a story. And, yes, there are exceptions. The original Dragonlance Chronicles, for example, turned a game into a good read - but, even then, they ran into issues. The infamous story of the dragon in the well, for example.
The thing is, however, that I feel that the division is one-way. If you are a fan of a TV series or a novel, there is no reason you shouldn't be able to set a game in that same universe. Licensed games have existed for years - the earliest one I played was the old Marvel Super Heroes RPG from TSR. The most popular one that I can think of is Call of Cthulhu.
Licensed games do a lot of the work for you - but sometimes you want to play in an unlicensed setting (or are dissatisfied with the licensed product). I use these same guidelines in situations when I'm dissatisfied with a game's rules set (I really like many of the concepts and ideas contained in Rifts, but I really hate the system).
What steps do I follow, then?
1) What Does The Setting Need Rules For?
Look at the source material and figure out what the rules need to cover. You wouldn't do a Babylon 5 RPG without starfighter combat, would you? (Sadly, they did.)
2) What Rules Don't You Need?
Crouching Tiger, Hidden Dragon won't need firearms rules, for example.
3) How 'Crunchy' Do You Want The Rules To Be?
I'm going to call out Rolemaster here - every single weapon in Rolemaster has its own chart for to-hit and damage (and critical effects). And it has charts and charts and tables of modifiers for special situations - so if you're fighting blindfolded at night in a swamp with one hand tied behind your back and your opponent is left-handed with a slight limp, Rolemaster has you covered rules-wise. Do you want and/or need that level of detail for your rules? Or will the XPG system work better for you?
4) Do You Already Have A Rules Set that Will Work?
The excellent Red Dwarf RPG can be tweaked to fit a huge number of settings. If you can't find it anywhere, Mean Streets uses the same system for Film Noir. Radz is post-apocalyptic. Santa's Soldiers is ... weird. Just look through Deep 7's offerings - anything described as 'XPG' will do the trick.
What about a generic system, such as Cortex - does it have what you need?
5) What tweaking will that rules set require?
You could do Babylon 5 with Traveller. Very easily, in fact. You just need stats for the various alien races. And their ships. Assuming your players are already familiar with the setting, that is ...
But if I were to use Silhouette to run a Shadowrun-style game, I'd need to create or find rules for magic. I'd need to create or find rules for non-human races. I'd need to create or find rules for hacking and the 'Net ...
6) Are Your Players Interested?
This is the most-often ignored step. You could be the biggest A-Team fan in the entire universe, but if your players aren't interested, the game will fall completely flat. Even if you've painstakingly tweaked Leverage to account for more guns and less witty dialogue, it won't work.
Another example? I've become a HUGE fan of the Shadows of the Apt series (Book one is Empire in Black and Gold). I have it about half-converted to run in Runequest II, but none of my players have read the books. Since they know nothing of the setting, I could just as easily run it as a D&D game, and they'd never know the difference.
7) What Do Characters Do?
Office Space would probably not be a good setting for an RPG.
Until Leverage, I'd have said that you'd never be able to do The Sting as an RPG. But now it's possible.
The point is: If you don't have at least one playable adventure, and the setting doesn't have hooks to hang further plots from, then why bother?