There are two sections on reactions in the three original rulebooks. Book I (Men & Magic) has a section on non-player characters, pages 12 and 13, in between the section on rolling up ability scores and the section on buying equipment. Obviously, this is because hiring mercenaries, torchbearers, and porters is seen as an important part of creating a character. The section on ability scores sets up the NPC section by defining how many hirelings of "unusual nature" may be hired, based on Charisma; ordinary men-at-arms are specifically exempted from this limit, but not from the effects of Charisma on loyalty.
The NPC section points out several things:
- The only NPCs you can hire outright in town are normal men (as mercenaries and labor) and low-level classed characters;
- Monsters, including higher-level humans found in the dungeon and wilderness, can be lured into service (limited by alignment) or charmed (no limit);
- Luring into service requires an actual offer, not just sparing the monster's life, although a monster that is subdued or surrenders can be offered treasure and lured into service;
- The 2d6 reaction table is introduced as a way to determine if a monster accepts an offer to serve;
- If hirelings or monsters agree to serve, a 3d6 loyalty check is made, which determines modifiers to morale rolls;
- GMs are given the option to use Chainmail's morale system or the 2d6 reaction table to check morale.
The second section on reactions is in Book III (Underworld & Wilderness Adventures,) near the bottom of page 12, just after a section on avoiding monsters. The setup here is that monsters will automatically attack or pursue any adventurer they detect unless an intelligent monster encounters an obviously superior group of adventurers. The rules then state that such intelligent monsters will act randomly and offers a simplified version of the 2d6 reaction table (three possible reactions, instead of five.)
What's clear here is that the reaction table is not meant to be used for most monster encounters; the reaction is predetermined ("hostile".) What happened in practice is that GMs started rolling more frequently for reactions, to make things more interesting; not every monster attacks immediately. Another change in practice, endorsed by Gygax in AD&D, is for negotiations with merchants.
There's also a change in the way alignment affects reactions, spurred by a change in the way alignment is interpreted. Originally, alignment was just a side or faction, which is why there were only two sides and a neutral, unaligned middle ground. Alignment limits which monsters will accept offers to serve an adventurer; there's no explicit guidelines, but my interpretation is that opposed alignments have a default reaction of Hostile (and will not serve,) while any any mixed alignment interaction will have a default reaction of Neutral. When alignment changed to a behavior guideline instead of a faction, GMs start using modifiers for specific alignments; there's a table of modifiers in the DMG, for example.
There's some more I could say about judging monster reactions, but I'd like to open this up to examples of how everyone actually uses the reaction rules. Do you roll for every monster? Predefine all reactions? Only roll for certain kinds of offers or behaviors?