In the latest Web DM podcast on “Making the Most of Your Monsters” Jim describes the old school 2d6 reaction roll about a half hour in and breaks down how to use the different results (here on Apple Podcasts or here on Spreaker, and should be up on YouTube soon). My favorite version is this one above from Moldvay’s 1981 Basic D&D rules.
Obviously these results can be adjusted with a + or - modifier if you think the monsters should be inclined one way or the other, or a reaction can be predetermined if they are on the same or opposite side of an ongoing war, etc. The paragraph preceding this implies that the PCs actions should be considered first before checking the monster’s reaction:
Put this all together and you see why PCs often kept a meal’s worth of rations and a small bag of coins handy to offer to hungry beasts or intelligent guards in order to influence their reactions.
