What does a character roll against to (for example) climb a wall, pick a lock, sneak passed a guard?

"Sneak passed a guard" - Bad example, it would be abilities v abilities.

What I mean is, when a character attempts an action, which does not have another character in opposition, what would be used as the resisting factor?

Thanks

You need to closely examine what the goal of the action is. If the goal is to get inside someone's house undetected, for example, there will be guards and security systems to defeat. To defeat an inanimate object, you are really contesting against the person who built or designed it. Assign a Faculty and Aptitude (security expert, for example) to the person who put the system together, and have the character contest against that. This goes for locks as well.

I think it a bit unrealistic to say "Stonemason Adams built that wall, and he's an expert, so the difficulty would be higher than Jim's." But I see what you are saying.

Having said that, it could be used for certain instances. But either way, I think it would be relatively easy to create some kind of a Table of Target Numbers.

Task                  Target
Difficulty            Number
Very Easy            5
Easy                    10
Average                15
Hard                      20
Very Hard              25
Almost Impossible  30
Impossible            35

Or something.

Character passions could still be used of course. If a loved one was held captive behind a 30' wall, then that wall would be less intimidating to the climber.

You could certainly do that, although the scale should be more like this:

Difficulty                    Target
Very Easy                    2
Easy                            4
Average                        6
Hard                            10
Very Hard                    15
Almost Impossible        20
Impossible                  24

This accounts for the fact that the very best someone can normally do is 18 (5 faculty + 5 aptitude + 8 action tokens). Add in 5 for the highest passion you can get, and you reach 23. So, 24 actually is impossible unless you use power tokens.

Thank you Brennan, we'll use this in our games.

Let you know how it goes.