NPC Stats

Since most NPCs play a relatively small role in the overall story, and since most NPC rolls are represented by the Challenge Dice the PC rolls against, NPCs stats don't need to be as detailed and nuanced as those used to define Player Characters. To help keep things simple for the GM, we've created three different schemes for NPC stats with varying levels of detail. When creating an NPC, pick whichever stat scheme works best for the role they'll play in the game. If you don't expect you'll need to know a particular NPC's stats during the game, don't waste your time coming up with them. You can always make them up on the fly if the need arises.

Extras

Extras are background characters who mostly exist to provide color, get in the way, and sometimes die (at the hands of either the PCs or the bad guys). Extras whose primarily function is to serve as cannon fodder are called "Mooks." In many cases extras don't need stats, but Mooks and other extras who exist to provide some type of (not especially challenging) opposition to the PCs at least need a way for the GM to determine how many Challenge Dice players roll against. So that's exactly what they get. Most Mooks just have two stats: A role and a Challenge Dice Rating (CDR). When the extra is doing something that fits the role (for example, when a PC is trying to punch a Nazi or spot a hidden ninja), the PC rolls against a number of Challenge Dice equal to the extra's full CDR. In all other cases, the PC rolls against CD1.
In situations where a PC's opponent would normally suffer physical or mental damage, each success lowers the extra's CDR by 1. When the CDR reaches zero, the extra is defeated. PCs who win a roll against an extra can divide their successes among any number of nearby extras with the same or lower CDR. For example, if Captain Belgium is fighting a group of CDR 2 Nazis and rolls 6 successes, he can knock out 3 Nazis in one round.

Bit Players

Bit players are probably the most common NPC category. They may play a significant role in a particular story or scene, but their time in the spotlight is short-lived and they tend to disappear from the campaign once they've played their part (though they can always return later if the story requires a minor character who matches their description). Examples of bit players include most of the monsters PCs encounter in a dungeon, the victims who ask the PCs to protect them from the bad guy of the week, and the eccentric expert who the PCs seek out for advice on solving whatever dilemma they're currently pondering.
Like extras, bit player have a role and a CDR, but their Challenge Dice for things outside of their wheelhouse are equal to half their CDR (rounded down) instead of 1CD. Instead of taking damage directly to their CDR, bit players have Hit Points and Trauma Points that they lose when they take physical and mental damage respectively. If Hit Points drop to zero or less, the NPC dies. If TP drops to zero or less, the NPC surrenders, runs away, or otherwise stops being a problem.
In addition to their role, CDR, Hit Points, and Trauma Points, bit player often have additional traits that work like Hooks, Tropes, Trademarks, or Special Effects. These can include both story rules (vampires have traits like "doesn't cast a reflection", "needs invitation to enter"), bonuses to certain rolls (a wizard with a +1d8 bonus when using fire magic), special abilities (rules for a snake's venom), or anything else that needs to be defined about the NPC.

Supporting Cast

Supporting cast members are NPCs who play a significant role in the story: recurring allies or rivals, level bosses, and nemeses. Supporting NPCs have the same game mechanics as Player Characters, but in many cases (especially if you're writing up the stats in an encounter description), you can streamline them a bit to save space and make things easier to find. Here are a few things you can do to slim down supporting NPC stats:

• Hooks: Only worry about hooks that are readily apparent (like the fact that the NPC has giant bat wings) or relevant to the scene or game (combat-related hooks for a boss monster, for example). You can fill in others if they're revealed as the game plays out.
• Tropes: The default Trope rating is 1d6, so assume that any Trope not listed has a 1d6 rating. Just record the ones that are higher or lower.
• Health, Nerve, Luck, etc.: Instead of including dynamic stats in the character description, use a method that lets you track the Health, Nerve, and other frequently-changing stats of multiple NPCs in one place. A standard form with spaces for the NPC name and relevant data is the traditional way to handle this, but you can also use a spreadsheet or other electronic record-keeping method if you're comfortable with the software and reasonably sure you'll have access to a device (and internet connection, if necessary) to run it during the game.
• Other Stats: Like with Hooks, stick with Skills, Special Effects, and other traits that are unmissable or likely to come up during play. If the NPC's role in the campaign grows to the point where you need to know more about them, you can fill in the details as they become relevant.

Player rolls against supporting characters work just like PC vs. PC rolls: each rolls the relevant die pool and successes cancel each other out. Whoever rolls better gets whatever successes are left over after cancelling out all their opponent's successes.

Extra & Bit Player Ability Checks

In most cases, the GM can simply decide whether an NPC succeeds or fails at an action that isn't opposed by a PC. Sometimes, though--when the NPC's success or failure has significant story consequences, when it's helpful to know degree of success or failure or the game mechanics associated with a roll, or when the GM honestly isn't sure whether the NPC would fail or succeed--an NPC ability check can be helpful. When this happens, use the NPC's Challenge Dice Rating as their Die Pool and set Target Numbers just like you would for a PC roll: 4-6 for actions that fall within the NPC's skillset; 6 if the NPC is out of their element; and 5-6 for everything in between. Determine what Challenge Dice they have to roll against (if any) normally.