This awesome blogpost from the old-school OSR (OSOSR?) has 20 rules questions for your favourite D&D-like. I thought it would be a good exercise to answer these questions for (my understanding of) AD&D by the book. This would be pretty much my answers for my upcoming (hopefully soon!) new AD&D campaign.
Ability scores generation method?
Method III: 3d6 six times, pick the best, repeat for each stat. Or the almost (with 0.5% accuracy) equivalent d100 table posted here.
How are death and dying handled?
If a hit brings your HP to exactly 0, you are unconscious. Even if woken up or healed, you are out of the action until you can get 7 days uninterrupted bed rest. If a hit brings you to negative HP, you are dead.
What about raising the dead?
That is possible with the raise dead or resurrection spell, contingent on passing a Resurrection Survival check. If you pass, you are raised, your CON is reduced by one, and your max. resurrection counter is reduced by one (your max. resurrection counter at character creation equals your rolled CON). If you fail the check, you are permanently and irrevocably dead.
How are replacement PCs handled?
All new characters must begin at 1st level, there is no taking over of higher-level characters with the exception of henchmen in the retinue of the deceased character. In general, characters are not "spawned" just because a PC died. Normally, 1st level characters are created in towns and villages, and can be retrieved there by the party. They don't just appear wherever is convenient. They also don't suddenly decide to go alone to a dungeon just to join a party that they don't know. But players may take steps to ensure that a replacement character is always nearby. They can employ hirelings and henchmen that they get to "take over" if a PC dies. Hirelings will become level 1 characters at the end of the session; henchmen retain their level (and this is the only way to start playing a character of a higher level.) Players cannot bring multiple characters from their stable along without there being a henchman-liege relationship.
Initiative: individual, group, or something else?
Group initiative. High-Dex individuals may fire missiles out of initiative order as described by the Dexterity rules, but only if unencumbered. Actions are declared before initiative is rolled!
Are there critical hits and fumbles? How do they work?
There is no such thing.
Do I get any benefits for wearing a helmet?
Yes: for the sake of speeding up the game, wearing a helmet is mandatory if you are wearing any kind of armor. (Otherwise every attack incurs a chance of the AC 10 head being targeted... and this DM doesn't want to deal with that.)
Can I hurt my friends if I fire into melee or do something similarly silly?
Absolutely. Firing into melee hits a random target among all the combatants in the melee.
Will we need to run from some encounters, or will we be able to kill everything?
You will need to run from some encounters, and you might want to do that before you are engaged in melee due to the free +4 attack the monsters would get against your back otherwise.
Level-draining monsters: yes or no?
Yes. Be careful and don't neglect to play and advance clerics.
Are there going to be cases where a failed save results in PC death?
Yes. If you have to roll a save you already messed up. Be glad that you get to roll.
How strictly are encumbrance & resources tracked?
Encumbrance should be calculated at the beginning of each session to determine movement rates. From that starting value, just keep track of additional encumbrance due to treasure that you picked up. In particular, don't bother updating the ENC value after every torch you use or potion you consume. If you want to drop something heavy to free up space, feel free to recalculate encumbrance.
Resources are also tracked pretty strictly. You should bring along enough food and torches to get you through the expedition, and those will deplete at a steady rate. However, water is handwaved if you have a waterskin and are not in a desert. Consider electing a quartermaster among the players to handle resource tracking for all players.
On the other hand, the replenishing of resources once back in town is not handled strictly: everyone must pay upkeep costs every month, and it is assumed that you can replenish your resources with that upkeep money. You spend money to get new "max resources" and the replenishing of these is abstracted into upkeep.
What’s required when my PC gains a level? Training? Do I get new spells automatically? Can it happen in the middle of an adventure, or do I have to wait for down time?
PCs level exclusively between sessions and they all need to train. Training costs 1500 gp per current level per week. It lasts from 1 to 4 weeks depending on the "grading" of that character. Grading is done after every session. Characters are assigned a grade of 1 (best) to 4 (worst) according to whether they have played their role well with respect to their character class and alignment. At leveling, all grades between the last level gain an now are averaged, this gives the number of weeks the character must train. Training requires a higher-level tutor, but characters with an average grade of 2 or better can self-train for double the cost. Magic-Users and Illusionists get one spell every time they level. They choose one non-named spell from the PHB list, roll chance to know, and if they fail they note this and try again with another spell, this repeats until they succeed. MUs and Illus can learn additional spells by finding scrolls and rolling "to know" (erases spell from scroll in any case).
What do I get experience for?
a) Recovering treasure from a dangerous place and bringing it back to civilization or a friendly stronghold. This includes coins, gems, jewelry, and magic items. With magic items, you can either give up the item immediately and receive a lot of XP for it, or you can keep the item and receive less XP. A "given up" magic item is assumed to have been sold to the black/underground market and effectively disappears from the campaign. b) Defeating monsters in combat. Note that all experience gains are reduced if the challenge required to get them was way lesser than the party's strength.
How are traps located? Description, dice rolling, or some combination?
Both are acceptable. A thief's roll to search for traps is in addition to stuff anyone (including the thief) can do by probing, observing, etc. The thief should still say what he does when he searches though, he doesn't just roll abstractly. However, a failed roll does not normally spring the trap.
Are retainers encouraged and how does morale work?
The morale rules are pretty extensive. They are described in the DMG. Generally, they work by rolling 1d100 against a fixed morale rating (for monsters, 45 + 5/HD), rolling over that rating indicates morale failure, the degree of the failure is determined by the d100 minus the morale rating. When to roll morale is also in the DMG (iirc, for example at 25% losses and at the loss of a leader. Some circumstances may add to the roll, making morale more likely to fail). Retainers of all types are very helpful for a successful mission and players are encouraged to seek them out in downtime.
How do I identify magic items?
Via experimentation or the Identify spell. Generally, NPC magic users will want 2x the cost of casting this spell for item identification.
Can I buy magic items? Oh, come on: how about just potions?
Yes, but you have to a) find them, b) the Magic User must be willing to sell them to you (which is in general not the case), and c) the price is 2x the XP you gain for selling them. There is no easy purchasing of magic items in downtime like there is for equipment. Bartering magic items is explicitly an option, but again not automatic.
Can I create magic items? When and how?
That is described in the DMG and PHB in much detail. Feel free to read the details if you are interested. In general, that will happen after you've gained some MU levels. BUT the magic research rules can be used from level 1!
What about splitting the party?
That's possible, but then players are asked to keep their focus and not get distracted while the other half of the party is handled. The most common case of splitting the party is someone scouting ahead.