Lock picking in role playing games is a bit weird as they are often presented as a technology in advance of real world locking tech. In fantasy gaming there has always been this vision of clock work locks that contain self resetting traps. This is not the case in the real world. You could imagine the legal repercussions of someone getting hurt by a trap in the modern world. There is also a terrible secret you will learn if you being picking locks yourself. Many modern locks offer little to no security.
There are all sorts of locks in the world; historical and modern. For gaming purposes Lets treat all pre modern locks as of two types; pin and tumbler and other. Pin and tumbler type locks are defeated by the art of picking. Locks that fall into the "Other" category are defeated by exploiting known design flaws. When we talk about picking locks in fantasy gaming are talking about pin and tumbler type locks generally. Opening handcuffs or cracking a safe in a pulp game would fall into the exploiting "Other" category.
Pin and Tumbler locks are those in padlocks or your door. You can imagine nearly everything in the fantasy realm is going to be pin and tumbler. Solomon does a great job of explaining these locks in his article, you should read it. Generally locks have two part pins that prevent them from turning. When the key is inserted the pins are raised to the exact magic height where turning splits the two parts of the pin. When you are picking you are manually raising the pins to this magic height. Normally they would spring down but because of the voodoo of tension and imperfections in the locks manufacturing tolerances picking becomes possible. Its fine if you just take this as black magic but understand the operation involves to processes. The application of tension through a tension wrench typically executed through the non dominate hand, is the first part. The second step is manipulation of the pins with a pick held in your most dexterous hand. This is not to say this cant be done one handed, it would be difficult or require a weighted tension wrench but would be possible.
For gaming obviously one roll can cover the entire lock picking experience, but you are here for depth. Rickel goes deep on shit like Phoenix Command, that's right Barry Nakazono I love you too! So here is a general series of steps to flesh out picking.
- (Optional) External Identification - Identifying the lock type and external search for traps. here we are looking to see if this is a known lock type that may have a known exploit. The rules for picking in most games are stacked against newbie rogues. Living in a small town with few locksmiths they are probably going to have a fair bit of understanding of the local locks and their faults. I would give 15 to 30 Percent bonus in situations like this. Visual checks for traps are going to have poor results but are also completely safe. When you roll this make sure its is Skill plus Intelligence.
- Internal Identification - Here we are identifying the number and type of pins we are playing with and looking for traps. There are things called security pins which increase the difficulty of the picking process. I would not allow taking 20 on a lock without doing an internal identification. Traps are another concern and performing an internal identification successfully will alert the rogue to their presence. On the other hand a fumble ( Or failure if your a nasty DM, are you nasty?) will trigger the trap. This is both a mental and physical process as you are probing the lock and creating a mental picture of its mechanism.
- (Optional) Disabling Traps - Here you are disabling any traps that are present. This can be both mechanical disabling and simply activating the trap in a safe fashion. Here I am going to deffer to the game master, if you think your player has a good idea of how to accomplish the task give them a bonus. You can have activating the trap require a fumble or just a failed check. I would give some sort of saving throw. You may be wondering about how this step is optional. The player may be in a hurry and just attempt open the lock. Its a mistake but give your players the rope so they can make their own noose..
- Actually picking the lock - Here you are manipulating pins and applying tension. You are welcome to apply a 5% penalty per security pin in the lock if you like, but in reality these things increase time to pick more than they do difficulty. In game terms that can be pretty much the same I understand, but there is a subtle difference. In D20 I would do something like increasing the time to take 20 rather than not allowing it. You can do a similar Bonus or Penalty if you have more or less pins.
Going back to those that skipped disabling traps. If they fail or succeed they are getting hit by the trap. I guess give a save maybe. If they fumble don't even give that. If you are playing in a system that allows for superior success you can think about having that be the requirement to not activate traps.
For those picking the exact same lock again and again its worth noting that I gets easier. This is due to something called binding order. A particular lock binds up in the same fashion because of its imperfections. If you open and lock again and again you are going to become accustomed to it. Game Masters please consider this when assigning difficulty.
Environment is another factor when dealing with locks of all sort. Locks in the outdoors have a tendency to wear heavily and be difficult to use. Anything submerged will be in even worse repair, perhaps rusting parts together. The other side of environment is the general affluence of the area. Wealth buys better locks. And also there is the consideration of what it guards.
I also wanted to call out another lock that is functionally similar to the pin and tumbler called the Lever or Mortise. These are locks that have a series of levers that lift up like a railroad crossing sign. When all are in the stop position they prevent the lock from opening. Picking these locks requires a different tension wrench and picking wire. You are pushing the levers up like an open railroad crossing sign and opening the bolt. Rather than a pushing of pins its a turn of the wrist and working through the lever stack again and again. I would not change the rules outlined for pin and tumblers as both can be modeled with the same rules presented above.
Other locks are going to be less common in your fantasy game but shackles would certainly fall into this category. These are easier to defeat and will almost always have some sort of flaw in design. If the player can identify the lock they can take 20 to defeat these sort of locks, excepting a combination safe lock. I would give the player a roll should they want to attempt these tasks quickly.
Combination style locks can be cracked with feel and mathematical processes. A Combination padlock is going to be easy enough but a safe is exponentially harder. The padlock may be more easily shimmed in truth though. This is a process of using a disposable piece of metal to depress the locking mechanism.