Perception

[Zarithar]

Noob question here.

I am confused about the way perception works. Lets say we have a secret door... not sure what the DC should be. Bob the Fighter has an active perception of 15. If I understand this correctly, I roll a D20 and then add 15... is that right?

 

[HarbingerX]

Passive perception is 10 + skill modifier - it is what you use if the player doesn't say they are using perception. Think of it as the PC will automatically notice anything equal or less than DC 15.

If the player says they are actively using Perception then have them roll and add their modifier (from your example I assume it's +5). This means they will roll between 6 and 25. While this may seem strange that they could roll less than their passive check, if it's for something they would have notice anyway, then you wouldn't have asked them to roll in the first place.

 

[Camelot]

Like Harbinger said, Passive Perception is what they notice without actively searching. When they actively search, they roll a d20 and add their Perception modifier (Wisdom modifier + 1/2 level + 5 if trained + other modifiers). Actively, they may miss something that they would have noticed accidentally, depending on the circumstances.

For determining DC, it matters what level they're at. At levels 1-3, DC 5 is very easy, DC 10 is moderate, and DC 15 is hard. They each increase slightly by level (you can find the list of average DCs in the updates for the DM's Guide on the Wizards of the Coast official site). On level 1, a +5 in a skill usually means that they are trained, or are naturally good in that skill (with a racial bonus), so they can (in the case of Perception) notice things that are technically "hard" to notice with a Perception check.

Hope that helps!