In solo RPG play, the distinction between NPC and PC is blurred. Normally in an RPG encounters the players decide how to react and the Game Master decides how the NPCs will react. Often the GM decides with the aid of a reaction roll which sets the tone of the NPCs attitude toward the PCs. But, in solo play these distinctions are less clear. The player may choose exactly how the PCs react but with the same person also running the NPCs there may be conflicts of interest. In these cases opposed reaction rolls may be use in keeping encounters fresh.

Here’s tool for paired reaction rolls in AD&D first edition. Note this is not a house rule but just a tool for use in arbitrating encounters between two parties in solo play. It may and should be dispensed with if context and prior events make is nonsensical. In AD&D there are a set number of NPC reactions possible when encountering PCs (DMG p63).

If both sides of an encounter make reaction rolls then a question come up is how do these pair. Well, the answer is: however you want. But, this then doesn’t reduce the temptation of solo player bias. This, of course, can never be completely eliminated. But here is a matrix to aid in adjudicating these reactions.

The general procedure is to roll for reaction of each party using all relevant modifiers (see DMG p.36/37), cross reference to get the reaction type. Each sides reactions are separated by a slash. Comma delimited reactions mean it’s a fifty/fifty roll which reaction it will be. For the positive reactions gift giving also implies an offer to join forces. There are some permutations that will result in stalemate (both parties decide to set spears for example). In those cases make a second round of reaction rolls with all reaction modifiers halved (first impressions are the most relevant). If stalemate occurs a second time then both sides attempt to evade.

You will still need to decide on the details of each reaction. For example an attack might mean immediate charge if in close proximity or it might mean missile fire if at a distance.

Obviously there could be many ways to skin the cat here. This is the one I came up with. I would be interested in seeing other approaches.