Hobbit on Quest
This is one instance where pulling more from 4e might have helped. Here's what the 4e PH has to say:Passive skills are poorly understood because they never actually put the rules down on how to use them. I'm of the opinion that this was intentional in order to allow each group to decide how to use them, despite JC's insistence on how they're "the floor" for those skills. I use the Mike Mearls of method of rolling a die against the PCs passive skill, allowing a level of randomness while still rewarding spending resources on passive bonuses. One nice thing about this is that you can then expand the passive skills to include the knowledge skills (arcana, history, etc.) so that you can roll a passive check to give out some info before the players think to ask.
4e Players Handbook said:Checks without Rolls
In some situations, luck does not affect whether you succeed or fail. In a calm environment (outside an encounter), when dealing with a mundane task, you can rely on sheer ability to achieve results.
When you’re not in a rush, not being threatened or distracted (when you’re outside an encounter), and when you’re dealing with a mundane task, you can choose to take 10. Instead of rolling a d20, determine your skill check result as if you had rolled the average (10). When you take 10, your result equals your skill modifiers (including one-half your level) + 10. For mundane tasks, taking 10 usually results in a success.
When you’re not actively using a skill, you’re assumed to be taking 10 for any opposed checks using that skill. Passive checks are most commonly used for Perception checks and Insight checks, but the DM might also use your passive check result with skills such as Arcana or Dungeoneering to decide how much to tell you about a monster at the start of an encounter.
For example, if you’re walking through an area you expect to be safe and thus aren’t actively looking around for danger, you’re taking 10 on your Perception check to notice hidden objects or enemies. If your Perception check is high enough, or a creature rolls poorly on its Stealth check, you might notice the creature even if you aren’t actively looking for it.
5e largely dispenses with the need for a player to Take 10 by giving the DM the authority to declare success/failure outright when the outcome shouldn't really be in doubt. The passive check description here is, I think, nice and clear - clarity of the rules being one of 4e's general strengths. And notice the idea of passive knowledge checks here as well, one of the reasons why I'm specifically responding to your post, Shiroiken.
One drawback to the whole Taking 10/passive score issue, however, is the rules for them developed in editions where opponents who were level-appropriate were likely to keep up with a PCs skill value, making Taking 10/passive checks less of a likely success. They were great for weaker opponents, relatively mundane challenges but deliberately accepted a half-assed result that left half of the d20's results, the higher half, completely out of reach. PCs couldn't count on a lucky die roll to find a particularly challenging skulker or trap. That doesn't seem to be as likely with 5e with more PCs likely to be proficient in perception compared to monsters significantly skilled in stealth, or vice versa. The passive perception score tends to do really well by comparison.