Here’s my take on AD&D surprise and initiative. It’s not “what I think the book says” but “what I want to do with it”. In the case of surprise, I’m doing it pretty much by the book.
You roll 1d6, I roll 1d6, for each side. If I roll 1 or 2, that is how many segments I am surprised. Same for you. It’s possible we’re both surprised, for example if I roll 1 and you roll 2, we are both surprised on segment 1 but then I recover and can attack you on segment 2, before any initiative is rolled.
Monsters that offer a surprise penalty change the probability space on the die. For example, a monster that surprises 4 in 6 really offers you a +2 to the chance on the die. This means if you are less often surprised, such as 1 in 6, then if you encounter a stealthy monster you would be surprised 3 in 6 (+2 more often than normal for you). Monsters that are surprised or offer surprise on a higher die should be switched to the standard d6 according to the probability, rounded down (a 6 in 8 surpriser is 75%, and would become a 4 in 6 surpriser at 66.66%, that is, one who offers a -2 surprise penalty to enemies). This maximizes the possible number of surprise segments to 6 (assuming a monster is entirely unsurprised and offers a -4 or more surprise penalty, and the PCs roll 6 on their surprise die).
Being invisible and silent offers a -2 to surprise. That’s the typical Elf/Halfling in non-metal armor who does not have to open a door to get to the monster. Anything more than that would have to be pretty spectacular (psionic distraction maybe?). So the practical maximum is more like 4 rounds, but that would be uncommon.
Individuals in a group who have high or low DEX modify the number of segments they are individually surprised. This can’t go below 0 (that is, gain surprise on the enemy if your side was surprised in the first place).
Every segment of surprise counts as a full round of fighting, except that spells cast must have a casting time that fits entirely within the surprise period. So if you gained three segments of surprise on the enemy you could cast a Fireball, but you won’t know ahead of time how many segments you have so that’s a gamble. If you only had 2 segments, it would carry over into the first main combat round and you’d waste those surprise segments.
I think this works because it offers a chance for scouts to do something cool, and for stealthy folks to perform hit-and-run tactics. A scout who approached and surprised a sentry would get his segments of action, but his party far behind wouldn’t arrive in time – they get to charge in on round 1 of normal combat at the earliest.
— Initiative —
We declare actions first. Each PC rolls 1d6 and the monsters do too as a group, modified by individual DEX reaction adjustment. Whoever gets lowest goes first. If you have a weapon, you use your Initiative roll or your weapon speed, whichever is higher. I’m ignoring the jiggery-pokery of splitting up multiple attacks because that’s just silly.
Spell casting during combat is tough. You start casting at the start of the round, so if you’re hit anytime before you finish then you lose the spell. You finish casting on your initiative OR casting time, whichever is higher. Spells with casting times longer than the round carry over into the next round. The same happens with weapon speeds over 10 (although I don’t recall seeing any of those).
If you don’t want to do your action, now that you know what order everything will happen in, tough luck. This represents the dude who runs into the building too early and his friend throws the grenade in and catches him accidentally.
Here we have a system that lets people who just want to move, potentially move very early (because they have no weapon speed or casting time). It accounts for weapon speed and casting time without making everyone go on initiative 6-20 on a 10-segment round. We have the unpredictability of people doing things that require cracker-jack timing (you can avoid that, by the way, by inserting a one-round margin of error. Just tell everyone to stand still while you cast that Fireball …). It allows for DEX modifier to Initiative, but this helps people with light weapons more than it does people with heavy ones. It makes magic weapons with lower Weapon Speed very valuable, but moreso in the hands of an already-quick wielder. There is a chance for spellcasters to spoil a spell because they got hit. There is no (weapon speed or casting time) delay math, just a comparison. It has everything I want in an Initiative system.
It requires that the DM practice restraint in coordinating his monsters. That is, they have a great ability to all act at the same time. The DM should not abuse this, since it’s only there to make his life easier when rolling initiative for 7 giants and 19 dire wolves. In most cases, don’t attempt things that require segment-level timing. If you do, err on behalf of the PCs and have some of the monsters fail to time it right. If it’s a “open the door right when I finish casting Fireball” situation, give a 3 in 6 chance for success (+1 bonus for lawfuls and -1 penalty for chaotics maybe, or a similar bonus or penalty for troop quality). But that’s all ad hoc, just do what seems right while leaning against the monsters in general.
This ties into the initiative system for charging in AD&D, but I’m not convinced it’s needed. I like having the info on the table for purposes of PCs reaching into mysterious fountains with whatever is on hand, but for combat it doesn’t matter. I’m thinking about letting weapons of 5′ or over length attack over the heads of the front rank into a melee as if they were missile weapons, with random determination of target based on size. You’d include the front rank of PCs in that calculation! I wonder if anyone would be interested. It certainly seems like a nice option. Maybe let those with high DEX use their missile attack adjustment to modify the hit determination roll up or down by up to the amount of the bonus. I wonder what to do about Halflings and Gnomes in the front rank – certainly these Small melee members would count as 1 and any Medium ones as 2, which means a front rank of two Halflings against an enemy front rank of two Orcs would give a roll of 1d6: Halfling L, Halfling R, Orc L, Orc L, Orc R, Orc R. That arrangement might make a high-DEX second-rank spearman safe in stabbing above his friends’ heads.
Also anything 2′ or shorter is considered “concealable” and includes Short Swords, Horseman’s Maces, etc. These are the ones you can keep under your cloak or in a robe without people noticing.