Fixed difficulties can make it hard to model tasks that require advanced training.
For example, if someone wants to open a lock, the GM might think "This is just an ordinary door in a house - the lock is nothing special. I'll make it an average task, 'cause the lock is just an average object." However, this would mean that an untrained person with a decent Dexterity could open the lock with a few tries - not very realistic.
D&D and many other d20 games deal with this by saying that some skills cannot be used untrained - if you've not invested your skill points in the skill, you just can't do it.
Me, I don't like that so much, as it adds another level of look-up: which skills are prohibited to untrained characters? It's not always intuitive.... and I promised I'd provide my solution. Well, it's a been a little while (ahem), but here it is.
Do we think this task requires special knowledge?Usually, this is a yes / no question, and the answer is clear.
For example, climbing is an innate skill of human (and other humanoids, depending on your game setting) - yes, skill makes us better at it, but anyone with sufficient mobility can climb.
Similarly jumping, running, and other raw motor skills.
Also fighting - we can all fight to some extent, even if that just means flailing the limbs wildly and hoping.
On the other hand, can you try electronic engineering without knowledge? Not so much.
When there is doubt about whether a skill needs knowledge and training, make a decision for the particular skill check, and move on. After the game, think the decision over - was it right? Don't get stuck on little things during the game itself.
Not knowing the topic imposes a penaltyOnce we've decided that special knowledge is required for a given skill check, then we think about whether you know that stuff.
That's easy - make a Knowledge check to know the topic. And because we've already set target numbers for skill checks, we only have to decide if the Knowledge is Easy, Average, Hard, Heroic, or Epic.
For complex systems and tasks (like retro-fitting a turbo to an engine, or deciphering a dead language), we can require multiple Knowledge checks, or higher success.
For example, "What are all these symbols?" is an Average task, "Knowing these symbols, what happens when they're arranged like this?" is an Average task, or a Hard task if it's your first roll (so you skip a stage of the process); and it's a Heroic task to establish that "I recognise this arrangement of symbols on sight."
Outside knowledge can be influential, too - in a modern game ,there's loads of information available almost all the time. Does the PC have a manual or some sort of instructions for the task, for example? Then we can allow this to take the place of a Knowledge check success, but probably at a slower pace.
But what does success mean? We're buying off penalties here. If you know little about mechanics, it'll be hard to fix an engine, but it's not impossible - a heroically intelligent PC might be able to figure it out with hard work and perseverance. So we're not changing the DC of the task - the task is the task, it will always have the same DC. What's changing is the PC's understanding of the task.
Setting the penaltyHow hampered by a lack of Knowledge will you be in this task? Again, because we've already set a bunch of standard adjustment levels using natural language, this should be easy to decide. Is it a Minor, Major, or Extreme problem? You can decide this from your own experience, or use the Difficulty level to give you a clue to what's appropriate - lack of Easy or Average Knowledge will generally be a Minor penalty, Hard would be Major, and Heroic or Epic would be Extreme.
So if the Knowledge check is failed, it may not be the end of your task attempt - you might only be faced with a Minor penalty. Or it could be Extreme.
For extended complicated systems, you might be able to work out things as you go along examining the problem. That'll let you make new Knowledge checks after starting. So you can start trying to buy down that penalty to something more manageable - all of which will take in-game time.
That's itDecide: does this skill check require special knowledge?
Set the Knowledge DC, or DCs for complex tasks.
Set the penalty for lack of knowledge.
Decide: do we allow new Knowledge checks as the task progresses.