I'm sure these rules have been discussed extensively already. I have read several forums with convoluted answers. So here are a few questions with examples. (All rules references will be to material on d20pfsrd).
1) The Perception skill mentions DC modifiers for Perception Modifiers. What about opposed rolls like Stealth? Is an opposed roll a DC and do modifiers like Distracted, Distance to Source, and Through a Wall become bonuses to the opposes Stealth roll?
2)Line of Sight...Is it assumed that every PC, NPC with its eyes open has full 360° 'line of sight? Are there any rulings or precedences on the reality that many creatures have limited fields of vision?
-Say a thief wanted to sneak by a guard. There is no cover. It is normal light. By Pathfinder rules, without cover or concealment, Stealth checks aren't usually possible. Does having ones back turned grant concealment to creatures behind them and allow them to make Stealth checks?
-And looking at Perception DC modifiers would the sneaking creature get a +20 because they are effectively invisible to the creature whose back is turned?
3) Per the Perception rules, Perception checks can be either reactive, made in response to observable stimulus, or intentional. It seems realistic that every reactive Perception roll should be made with the Distracted penalty (if your not actively, intentionally perceiving, you're doing something else and so should be considered distracted). This is mostly opinion as I feel someone actively perceiving should be better at it than someone not actively perceiving.
1. Yes; note the entry for "Notice a creature using Stealth" has "Opposed by Stealth" under the column "Perception DC". Modifiers apply as usual.
2. Characters aware that combat is occurring are assumed to be constantly looking around them for threats during the course of the battle and are thus able to see anything unconcealed within line of sight. This is a conscious design choice to simplify combat without having to keep track of visual cones for each creature every time it does something.
Flanking represents the idea that threats on both sides are causing a creature to be unable to defend itself as effectively due to its inability to track multiple threats at the same time. A creature with actual all-around vision (such as a gibbering mouther) usually has the additional benefit that it cannot be flanked.
Outside of tactical combat, facing isn't really addressed. I generally play it narratively, allowing a rogue to creep up behind someone and pick their pocket so long as the person isn't aware that they ought to be looking around.
2a. If the guard isn't in combat, isn't aware of the thief, and isn't constantly looking in the thief's direction, sure. If the guard is particularly intent on looking in a particular direction (say, it's guarding the way into a vault and assumes that threats could only come from outside), a thief taking advantage of this fact (continuing the example, by somehow already being in the vault) would have an advantage--I'd treat the guard as distracted.
2b. The thief is not invisible, though, so a guard might catch the thief in her peripheral vision, while turning in response to a (possibly unrelated) noise, etc.
3. A character just standing around doing nothing, idly chatting with someone, walking down the street, or otherwise unoccupied would not be considered distracted. A character lost deep in thought, reading a book, having a heated debate with someone, or otherwise engaging in activity that demands her full attention would be considered distracted. An active Perception check simply means that a character is specifically attempting to look for things, particularly things that aren't in plain view.
There's some table variation here, but to my understanding, the primary advantages to actively taking an action to Perceive is that you are decidedly not distracted; you can try again and again; and, depending on what in particular you're doing, you may negate penalties that would otherwise cause the character to miss a detail. For example, actively searching a room might involve looking behind curtains, knocking on surfaces to listen for hollow sounds, or even sniffing for unusual scents one might not notice in passing. These can produce observable stimuli (remember, you only get a reactive check if there is an observable stimulus), change an unfavorable condition to a favorable one, or change a fine detail into a much more noticeable one. There's a lot of GM discretion involved here.
Note that depending on the action, a player actively using Perception may end up taking the distracted penalty against things they aren't really looking for! A character actively rifling through messy file cabinets for a particular document isn't really paying attention to the sound or smell of inhaled poison venting into the room....
I'm sure these rules have been discussed extensively already. I have read several forums with convoluted answers. So here are a few questions with examples. (All rules references will be to material on d20pfsrd).
1) The Perception skill mentions DC modifiers for Perception Modifiers. What about opposed rolls like Stealth? Is an opposed roll a DC and do modifiers like Distracted, Distance to Source, and Through a Wall become bonuses to the opposes Stealth roll?
As mentioned, the DC to notice someone using stealth is the stealth check's value. [That is the opposed roll.] All perception modifiers apply normally.
2)Line of Sight...Is it assumed that every PC, NPC with its eyes open has full 360° 'line of sight? Are there any rulings or precedences on the reality that many creatures have limited fields of vision?
As mentioned, there is no facing in PF, so as long is there is a sight line between them, they have a chance to spot it.
-Say a thief wanted to sneak by a guard. There is no cover. It is normal light. By Pathfinder rules, without cover or concealment, Stealth checks aren't usually possible. Does having ones back turned grant concealment to creatures behind them and allow them to make Stealth checks?
If the thief can move from cover to cover, then a successful stealth check opposed by the guards perception allows him to pass unseen out in the open. Likewise, a stealth check can let him sneak up and attack once before the guard can react. [I.e. the guard is flat footed and the thief gets sneak attack die.]
-And looking at Perception DC modifiers would the sneaking creature get a +20 because they are effectively invisible to the creature whose back is turned?
No. Unless they are actually invisible, they don't get that bonus.
3) Per the Perception rules, Perception checks can be either reactive, made in response to observable stimulus, or intentional. It seems realistic that every reactive Perception roll should be made with the Distracted penalty (if your not actively, intentionally perceiving, you're doing something else and so should be considered distracted). This is mostly opinion as I feel someone actively perceiving should be better at it than someone not actively perceiving.
Distracted means you are focused on something else. A guard standing guard is not distracted. A spellcaster who is casting a full round spell is distracted. The GM usually decided if the distracted modifier applies.
Spending an action to perceive lets you search for treasure, check to see if there is a trap on the door, or hear if someone is nearby. You get to try to perceive some things that you cannot get a reactive check for. You get to retry if you spend an action. You get to spot an ambush where the enemy is already set up.
/cevah