SURPRISE (c.f. PHB 102)
Chance to Surprise Others
A party uses its least stealthy member’s chance to surprise others. So if a dwarvish fighter (surprises 2/6), elvish wizard (surprises 4/6), and a human ranger (surprises 3/6) are traveling together, the entire party surprises 2/6. Generally groups must be separated by at least 90’ in order to merit its own chance to surprise others.
Chance to Be Surprised
A party uses its most alert member’s chance against being surprised by others. So any party with a ranger in it has a -1/6 chance to be surprised. Sometimes it is impossible to be surprised, for example, when one party is observing another directly or is prepared for the encounter.
Common Modifiers to Surprise Others
Racial Abilities: Elves and Halflings gain +2/6 to surprise others. This bonus does not stack with silent movement. This bonus is lost if they must open a door. The bonus is lost if they wear any armor other than leather, dragonhide scale mail, or elvish chain. Contrary to the PHB, it is NOT lost if they are in someone else's company (although the entire group uses the lowest chance to be surprised -- this provision just lets elves hang out with rangers or invisible mages).
Silent Movement: Silent movement grants a +1/6 chance to surprise. Thieves can move silently as a class function. Certain spells or items also give a chance to move silently.
Invisibility: Being invisible gives a +1/6 chance to surprise. A thief hiding in shadows is effectively invisible.
Ranger Class Functions: Rangers get a +1/6 modifier to surprise others.
Ambush: Setting an ambush or other general circumstances can grant a +1/6 modifier.
Additional notes from elsewhere in my errata:
Surprise Bonus: At level 5, a monk is -1/12 to be surprised. At level 9, a monk is -2/12 to be surprised. At level 13 a monk is -3/12 to be surprised and at level 17 a monk is -4/12 to be surprised.
ELVES & HALFLINGS
Surprise: Elves retain their bonus to surprise when near humans and other loud types and when wearing elvish chain mail (but no other metal armor). However, note that a group of characters uses the least sneaky member's surprise check. Therefore, an elf (4/6 to surprise), a ranger (3/6), and a human fighter (2/6) surprises foes 2/6. Elves still lose their bonuses to surprise when they must open a door, although thief skills/magic could help to mitigate this (just as it could for a human). Basically, this allows elves to move with thieves or rangers as useful companions; for example, an elf (4/6 to surprise), a ranger (3/6), and a human thief that passed his move silently check (3/6) surprises foes 3/6. Provisional: This rule might change if it is "broken" in a way that I did not foresee.
Surprise: Surprise is important to thieves as it must be obtained to make a backstab and it can help to avoid detection. Being silent (passing a Move Silently Check) gives +1/6 chance to surprise. Moving immediately from hiding (passing a hide in shadows check) or invisibility gives an additional +1/6 chance to surprise. Most characters have a base 2/6 chance to cause surprise, although this can vary by circumstance and creature.
HEAVY (or fairly bulky armors): No bonuses to reaction in surprise situations or to initiative
(ex. from high dexterity)
FULL: No charging possible; Automatically lose initiative vs. non-encumbered foes; -3 penalty on surprise reaction rolls; -2 penalty to AC