Stealth is weird. Sneaking around has always been an odd point of contention at the tables I've played at. Perhaps it's my experience, but many of the GMs I've played with seem to not enjoy the idea of players sneaking through their entire enemy base and stealth killing the prepared final boss. Now, I do love me the epic final boss fight as much as the next guy. But, it is pretty lame when you and your fellow players go through all the planning, actually execute the stealth procedure, and as you get to the final boss, the GM simply squashes the stealth in a metagamey way. It removes the idea of choice and consequences if all of options have the same outcome.
Once thing about stealth that I don't like is the binary nature of success and failure people seem to have with it. If you (or chances are, your heavy armored fighter) fail your stealth, then that's it. Suddenly, everyone in the area is alerted to your presence and it's time to draw swords. When failure is that binary, there is a tendency to simply skip it and just go in swords blazing. Especially when the players probably have the damage and spells to just bust in and slaughter everyone.
Finally, even if players properly execute the stealth mission, if it's not run the right way, stealth can be super boring with an unsatisfying end to it. Stealth kills are great and all, but admittedly, it can be a bit anticlimactic to slit the level 10 Warlord's throat in one go without the fanfare and hooplah of a final fight. And stealth kills are also a bit hard to wrap one's head around, especially with the game concept of hit points. Should a level 1 fighter be able to one hit kill a level 10 fighter if the latter is completely unaware? Is that a problem, or is that fine?
How I Approach This
So, before I get into the rules, I want to get into the mindset of stealth and espionage. I feel that with sneaking, the journey is more important than the destination. So if you have a precious boss fight you want to throw at your players, just get rid of any attachment to it, because they will die. Really, it's a good idea not to be so attached to your NPCs in the first place, stealth or not. So let them slit the wizard's throat if they succeed and never get caught. And if they fuck up hard, then you have a great climactic battle ahead of you.
Instead, focus a bit more on the stealth aspect. Have a lot of close calls and snags that the players run into. That makes the sneaking action much more interesting than just dodging guards effortlessly. Have them balance over some bandits eating their meal, or their disguise being put to the test by a crowd of soldiers. The tension and near misses are the most interesting parts. You wouldn't have a dungeon with nothing in the rooms. That'd be boring. Get creative.
Also, we should take a page out of different movies and video games and not have stealth be so binary. If a person fails their sneaking roll by a slim margin, I say have the guard hear something suspicious and walk towards the source of the nose. It opens up some more options that the player can try and attempt to use to diffuse the situation. Does the player kill the guard, or move to a different spot, or stand perfectly still? Makes things a bit more interesting and it helps to extend out the stealth sequence and make it a worthwhile method to the players. I also find that in more lethal and visceral games like LotFP and ACKS, players are more incentived to go quiet for fear of dying.
Speaking of lethal, how do we handle stealth kills in a game with ever increasing HP? For me, I think I'm okay with the players being able to do take downs, but there are things to consider when killing a guard. First is the sound a dead person makes, whether it's the scream or the body hitting the floor. Someone may hear that if the killer doesn't act quietly while he murders someone. Second is what to do with the dead body? If you don't dispose of it, then you run the risk of someone discovering the dead body and alerting their friends. So as a player, you have to decide if killing the guard is worth the trouble.
Of course, if the players get discovered, then the stealth plan may go out the window. Realistically, it doesn't have to. Consider the Metal Gear Solid games. It starts with Infiltration Mode, where you are sneaking around and not getting caught. When you are caught by a guard, you get into Alert Mode. As long as they can see you, they will attack you. Once you stay out of line of sight for a time, they get into Evasive Mode. They still know you are around and are actively search for you in the more obvious spots. After some time, the game will go into Caution Mode. They have lost sight of you and everything is like Infiltration Mode, except now they are much more active and aware. They will act smarter, keep a keen eye out, and travel in pairs or groups. So, when I was making these rules, I decided to use that for my game. I believe that it can be used to great effect to give stealth different layers of failure and success, and it helps the players find an alternate way of sneaking to continue or escape.
I do have a set of rules I've used a couple of times to good effect for sneaking around. It seemed to make things better while still remaining simple. Once I'm able to post them up, I'll get them in. I can probably do it tomorrow, or even tonight. How have you all run stealth situations and what has helped you in running them?