Let’s say your players need to retrieve a heavily guarded item in a fantasy RPG scenario. How should you go about running that?
First thing: establish the location. You’ll need a detailed map of the place where the item is kept; find a map online or create it yourself. Place the item deep within that location.
Then, populate the location with traps. The item itself may have spells on it, or the room may have spells, or you may have mundane mechanical traps around the room (pressure plates that trigger poison darts, pit traps, etc.).
Finally, add guards. Make sure the item is guarded by intelligent enemies.
Let the PCs learn about all this, then let the PCs plan.
Note: You may need to limit their planning. I’ve used a kitchen timer, set to 30 minutes, and told the playersthat something bad will happen if they don’t commit to a plan before the timer goes off. The “something bad” depends on what’s going on in your plot; perhaps the PCs are discovered, or another enemy makes its move, or the item in question is moved, or security is beefed up.
Then, when the heist goes off, ensure that enemies react intelligently. The guards should be alert. They should call for help. They shouldn’t just stand there and attack the PCs until death; they should retreat for backup if possible. The guards should also have their own options for locking down the location and otherwise preventing the PCs from getting away.
Have you run a heist before? Please post your experiences in the comments!