One of my absolute favorite tricks in Leverage is that there is no obligation to stat the opposition from the getgo. Structurally, there are certain questions you need to answer about the mark and such, but the mark isn't really the opposition. I'm thinking more about characters like Sterling, the ones who can really give the players a run for their money and who make for interesting challenges. While the GM _can_ write them up, the system doesn't require it, and in fact offers a much more elegant solution.
The trick to this revolves around the primary use for complications (situations where a player rolls a 1). Complications give the GM currency which can be most easily thought of as narrator plot points. The GM can use them to introduce twists and complications in the form of slapping new descriptors down on the table. So, for example, let's say the players are casing a joint and they produce some complications. The GM might use those to say "Ok, there's an Insurance Investigator checking the place out too, that might be a problem". And if that GM is me, then he picks up a sharpie and writes "Insurance Investigator d8" on a post-it note and puts it down on the table. That's now in play, and the GM will pick up that d8 any time the Insurance Investigator comes up to mess with the players (and a clever player who finds a way to leverage the investigator might be able to pick it up too).
That's a good start, but where it gets fun is that the GM can add to it as he gets more complications. Let's say that this investigator is in a really good scene, and based on how it went (and the complications I have to spend) I add "Sees more than he let's on d10" to his post-it. Later on he ends up in a scrap on the player's side and one of the _players_ spends some plot points to add "Old Army Buddy d8" to the note.
The net result is that if an NPC is interesting enough, he will develop stats over time that emerge organically from play. This is pretty cool, and to come around to the point of this post, it can be turned around as a fantastic way to handle monsters in an investigative game.
See, the thing about Supernatural's monsters is that a lot fo them are throwaways. There are certain recurring types (demons and vampires, for example) but a lot of them are just some familiar-sounding name out of the mythology of your choice. Now, it's totally possible to build a monster in advance based on an idea, but that's not the only way to do it. It's entirely possible to build a monster from it's _effects_.
To illustrate what I mean, imagine the monster's stats as a blank sheet. As you start the adventure, you describe the gruesomely mangled bodies of the victims. With this point, you have revealed something about the monster - whatever it is, it's capable of making injuries like this. So you note down "Monstrous Claws d8" or "Heavy Cleaver d10" or whatever caused the wounds. If you want to leave it uncertain, then "Monstrous Claws? d8", with the question mark indicating that you might refine the descriptor later.
Later on as they talk to the Sheriff about what he saw, the Sheriff talks about unloading his revolver into the thing's back and it not even flinching. Slap down "Bulletproof? d8" on the sheet, leaving your options open, Maybe it's a ghost, maybe it's heavily armored, maybe it just shrugs off gunfire. When you get a better idea, you can scratch out Bulletproof and write down something more precise.
Sometimes information might be wrong (bad witnesses or the like) so feel free to note that with extra question marks ("Can fly??? d8") so that you know which information you can ditch if it ends up contradictory.
Continuing this over time you'll find yourself creating a complete picture of the monster while your players are doing the same thing. In effect, their investigation is your monster creation process. At some point it will all fall into place (for you or them) and all you'll need to do is slap on a name (and for that I really recommend having a list of monster names on hand in advance).
Not every game will suit this approach, and it definitely is a better tool for the GM who likes to discover things while describing them, but if you need to pull a session out of the air, this lets you do so with only the barest outline of a plan, and build it as you go.
[↩]1 - In my house rules, complications are a tad more potent than they are in base Leverage rules, starting at d8 rather than d6, on the reasoning that since d6 is the default die (that is, the die you roll when there's no relevant descriptor) the GM has an infinite budget of those.
[↩]2 - Note that the player has just done two useful things there - created a connection to the character AND given him something useful in a fight that the player can add to his pool in the fight.