Surprise

When two groups meet by chance in classic D&D games, there is a chance for each to surprise the other. Breaking it down, what factors might be going into this?

• Lack of awareness: One side might have lost attention - sleeping, eating, reading, sexing. One side might be blind and deaf in its environment, like a lightless swimmer under dark waters.
• Lack of concealment: One side is giving itself away - taking light into the dark, noise into the silence, laughing, gambling, brawling.
• Exceptional awareness:  One side might be waiting by the door to ambush oncomers all day and all night. One side might have ESP, or heat vision, or elven ears.
• Exceptional concealment: One side might be hiding behind a rock, on the ceiling; invisible.

The typical situation of a party going into the dungeon is gross lack of concealment, with naked lamps and clanking armor; but high vigilance; forging on into unknown territory.

The typical situation of bored watchmen is of greater concealment, but lower vigilance. Undead watchmen are more vigilant. Gambling, shouting watchmen are less concealed.

Nope. No dice roll needed.

Most surprise situations can be resolved by detailed enough setting notes about what the guards are up to, whether the vampire is stalking prey or playing the pipe organ, whether the spider is hiding in the webs or scuttling along the floor. For situations where the DM wants more spontaneity, a monster vigilance roll might be needed to see whether the monsters are in a position to ambush, or are giving away their own position. The chance to spot the ambush can be handled as a passive perception check (spotting, plus listening if the ambushers are moving). If the ambushing party has a hide skill and successfully uses it, the perception check gets more difficult.

To determine the enemy's disposition randomly, roll 1d6 for each individual, up to 5; the remaining individuals have similar profiles to the 5 rolls already made, in the order rolled.

Undisciplined groups and individuals:
1: vigilant; 2-3: distracted; 4: distracted and making noise; 5-6: asleep.

Disciplined groups and individuals: 

1: ambushing already; 2-3: vigilant; 4: distracted; 5-6: asleep (at least one member of a disciplined group of 2 or more will be vigilant)

Predators:
1-2: ambushing already; 3-4: vigilant; 5-6 asleep.

Inhuman: 

1-2: ambushing already; 3-6 vigilant.

Non-intelligent:
Vigilant to the range of their senses, unless in a situation to ambush (such as an ooze dropping from above.)

Example: Our group approaches a right turn in the passage, torches blazing. 100 feet down the corridor is an open guardroom where three undisciplined kobolds are lounging. Will the kobolds hear the clank of armor and see the reflection of light on the wall? The roll for the trio is 1, 6, 6, and the signs of the party approaching are pretty blatant, so the one vigilant kobold notices without having to roll, and scuttles over to shake and wake the other sleeping two.

This makes a much quieter noise (but can't be seen)  so I give the two party members in front a secret roll on their d6-based listen skills, one has a listen skill of 1 and the other has 2. The one with 2 rolls a 2 and hears some scuttling, faint groaning and whispering ahead and to the right. Round 1 is over.

Round 2, the party stops, hides the torch behind a shield and sends their infravision-having, non-clanking dwarf to look down the passage. The vision only goes 30' so he sees nothing but passage. His listen skill is also a bare 1, but he rolls it, so he hears some more noises of blades being drawn, buckles being buckled, and footsteps. The kobolds are arming themselves. At this point, if a party member with a fast move and a torch were to run screaming ahead, he would probably catch the two dressing kobolds, if not the vigilant one, by surprise. But no, they go cautiously. And the kobolds get one more move, unheard, to set up their ambush in the corner of the guardroom just right of where the passage comes in.

After a couple of rounds creeping down the passage in full defense mode, the party reaches the entrance of the guardroom, then state that they will swing out quickly and attack to the sides. Because they are wresting surprise back from the kobolds (at some risk to themselves!) I deny the kobolds the surprise attack they would have gotten, and normal initiative is rolled.

If the players had not shown guts like that, though, I would have judged the kobolds to be perfectly hidden behind the corner, so the front right member must make two successful rolls to "notice detail" (my proxy skill for reacting quickly to a visual stimulus) in order to deny the surprise attack prior to initiative.

The moral? Time is not always on the party's side; and the cautious approach won't always give the best results. This insight is the best cure for over-cautious party creep I know.