I thought about this a bit more last night, and had a few ideas on when I would require a roll for each thief skill:
This works when you can devote a whole turn to it. The roll determines if you crack it in the first round or not. As a DM I might put a level cap on some locks, such that only a thief of the listed level or above could open them. The players might also like this for locking up their own stuff…
I might also just require a check every turn. I like the idea of a thief laboring at a lock for a couple hours while the rest of the party gets increasingly nervous about wandering monsters and those noises they keep hearing.
This works if you take a turn for each 10x10ft area. If you check hastily in one round, as in combat, you would roll. Removal works the same way.
This works as long as your target is not aware of you, as in a crowd or when you are sneaking. Otherwise you make a check to make something “disappear” off a table in front of someone, for example.
You know, as a thief, how to move without making noise, assuming your leather armor is properly oiled and you are carrying anything noisy. You make a roll if you want to move faster than 1/2 your speed.
This succeeds unless, like above, you want to move more quickly.
Hide in Shadows
This succeeds if you have time outside of combat to find a good place to hide, but if you are trying to “duck out” in a combat situation, behind a wall or into a dark alcove, you need to both be within your movement speed of a hiding place and roll against this skill.
Success means that your opponents have lost sight of you at the end of your round.
If it’s loud enough to hear, you hear it. You roll to get a general idea of what it sounds like (voices, footsteps, etc), which could be followed by a knowledge check for identifying, for example, what kind of monster makes that noise.
All of these combined, I hope, would make master thieves better at pulling off really cinematic thief stuff, while beginner thieves who are cautious can do much of the same stuff.
Players would still have to decide how to use their skills, and the ability to move silently or hide somewhere does not make you truly invisible. A vigilant opponent looking your direction is going to see you, so a clever thief will need to consider line-of-sight … and also things like how noisy it is when an armored sentry hits the ground after you snipe them.
As it stands, nobody in my player group chose to play thieves. I hope this can change that.