As I was watching it, I kept thinking how easy it would be to work some of that in to Gumshoe, the system used in Esoterrorists and Trail of Cthulhu.
In the Gumshoe system, one could argue that attacks, particularly gunfire, aren't quite as damaging as they could (or should) be. Your typical character can take a gunshot or stab wound and be just fine. That's great if you're running a cinematic adventure game, but most of the rest of Gumshoe is geared towards bookish investigator types and gritty realism. So, a method of situationally boosting your combat effectiveness might not be a bad thing. Putting in a Holmes-inspired system is overall pretty simple and elegant.
Here's how it works: For each NPC, the GM comes up with 1 or 2 weaknesses, and the corresponding skills that could be used to ferret them out. In the film, you might argue that Holmes only used Medicine for this (or only used Evidence Collection, depending on your point of view), but for a campaign it'd be important to spread it around so that no single Investigative Skill suddenly becomes a combat powerhouse.
For these weaknesses, the GM assigns one of the following opportunities:
- The weakness represents a way to incapacitate, or more severely harm the NPC. On a successful hit, you may make an appropriate skill spend to get +2 damage.
Example: A //Use// of the Medicine skill reveals that the foe has a trick knee. If you hit the foe, you may immediately //Spend// a point of Medicine, Scuffling or Weapons to further injure the knee. If you do, you deal an extra 2 points of damage.
- The weakness represents a whole in the NPC's defenses. If you //Spend// a point of an appropriate skill while attacking, it provides a +2 bonus to hit.
Example: A //Use// of the Outdoorsman Skill allows you to recognize the specific camouflage pattern the sniper is wearing. Armed with that knowledge, it's easier to tell him from the background as you return fire. When shooting you may now spend a point of either Outdoorsman or Sense Trouble on each shot, and if you do you get +2 to the attack roll.
- The weakness represents a limitation to the NPC's attack capability. An appropriate //Spend// can increase your own Hit Threshold, versus their attacks only, by 2. Whenever you do this, the affect lasts until your next action, and if you wish to maintain it beyond you must continue to pay Spend points.
Example: A //Use// of Weapons has revealed the weakness of his fighting style. He over-relies on a handful of moves, and always telegraphs his attacks. You may //Spend// Weapons or Athletics to boost your Hit Threshold to 5 against his next melee attack.
If the GM is feeling adventurous, and really trusts the players, you could also do this more free-form. Allow the above abilities to be something the PCs can invoke with their spends, and in fact make up on the spot, at any time. Just say to your players: "At any time you can spend a skill point to create a weakness in any NPC that doesn't already have one. Spend an appropriate skill, narrate in the NPCs flaw, and it becomes part of the character." There's some potential for abuse there, in that a player could build their PC in a very min-maxed way and then always give every foe a flaw that can be exploited with their one particular maxed-out skill. If you think that's likely to happen with your group, then fall back on the first system I described, where only the GM defines the weaknesses.