First, I want to direct people's attention to U-The Game of Stories whose creators contacted me about how this rules-lite RPG was great for duets. Here is a review for the game on RPGNow. I went through the game and have to say it is an excellent system for duets, and probably play-by-post as well, because it is simple, intuitive, and story focused. You want your rules to be intuitive and capable of quick resolution so you can get back to the real action — roleplaying.

This though leads into one of the fundamental stumbling blocks for successful narrating of duets: adjudicating roleplaying or story interactions. What I mean by this is how do narrators decide what happens when a PC does something that changes their relationship with a NPC or the setting. A lot of RPGs tackle social mechanics by changing their combat system into a social combat system! I've always abhorred this approach because it usually reduces roleplaying and always falls flat in execution. However, many narrators struggle with solid adjudications on the fly or have a hard time figuring out in an honest, realistic way how one or more NPCs might react to the PC's actions. This isn't an easy skill and it does come easier to some narrators than others, but I believe anyone can be an exceptional narrator if they can do two, very difficult things: relax and think. Narrating even in a duet can be tense, and relaxing and thinking aren't natural functions when you're tense. Thus we're going to use decision trees as a tool to help narrators until they can internalize the thinking process.

Decision Trees

We aren't using decision trees precisely as they are intended as our plan is to guide a narrator to making a decision. The goal is to break down the decision making process to a number of choices and when an answer isn't clear to use probability to make the decision.

So let's take something that happens a bit in duets: the PC sends a NPC or group of NPCs on an independent mission. The narrator then has to determine if the NPCs succeeded or not. Often this is pretty easy, such as the PC sends a NPC to find an inn for the party to stay that night and the narrator rules that the NPC succeeds. Most situations like this are pretty easily decided by all narrators, but the tricky areas are more complicated.

Decision Tree: NPC Mission 1. Is this mission something the NPC(s) should easily be able to handle?

Yes: The NPC should succeed in the mission, so go to #4.
No: Go to #2.

2. Is this mission pretty much impossible for the NPC(s) to handle?

Yes: The NPC should fail on the mission, so go to #5
No: Go to #3.

3. Determine the probability of the NPC(s) succeeding and roll that on a d%. If the odds are in the NPC(s) favor, use 75% as a benchmark. If the mission could go either way, then use 50% as a benchmark. If the mission is weighted against the NPC(s), then use 25% as a benchmark.

Success: Go to #4.
Failure: Go to #5.

4. The NPC(s) succeed in the mission. If the level of success matters, then roll a d6 and a 5 or 6 signifies a tremendous success by the NPC and a 1 or 2 signifies a success with possible complications.

5. The NPC(s) fail in the mission. If the level of failure matters, then roll a d6 and a 1 or 2 signifies a horrific failure and a 5 or 6 signifies a failure that might be salvageable.

Here's an example: a PC sends a NPC retainer to a neighboring dominion to arrange a meeting for her with the local lord.

1. Is this mission something the NPC should easily be able to handle?

Yes: The NPC is a bard with a high diplomacy/persuasion skill and the PC previously has had friendly relations with the local lord. There is no reason that the NPC won't succeed in the mission, so go to #4.
No: The NPC is a bard with a high diplomacy/persuasion skill, but the local lord is suspicious of the PC's intentions. Go to #2.

2. Is this mission pretty much impossible for the NPC to handle?

Yes: The NPC is a barbarian with intimidation as his only social skill and the local lord is hostile toward the PC. The NPC should fail on the mission, so go to #5.
No: The NPC is a barbarian with intimidation as his only social skill, but the local lord is interested in learning more about the PC. Go to #3.

3. Determine the probability of the NPC succeeding and roll that on a d%.

a. The PC sent an expensive gift with her retainer and the local lord views this as an opportunity to learn more about the PC, so the odds are in the NPC's favor; use 75% as a benchmark.
b. The local lord is suspicious of the PC but really things could go either way; use 50% as a benchmark.
c. The PC was once seen in the company of bandits who are wanted by the local lord for crimes. The mission is weighted against the NPC, so use 25% as a benchmark.

Success: Go to #4.
Failure: Go to #5.

4. The NPC succeeds in the mission. If the level of success matters, then roll a d6 and a 5 or 6 signifies a tremendous success by the NPC (the local lord not only agrees to a meeting, but also throws a lavish feast for the PC) and a 1 or 2 signifies a success with possible complications (the local lord agrees to a meeting but sends an "honor guard" of soldiers back to escort the PC to the meeting).

5. The NPC fails in the mission. If the level of failure matters, then roll a d6 and a 1 or 2 signifies a horrific failure (the local lord has the NPC arrested and thrown in the dungeons) and a 5 or 6 signifies a failure that might be salvageable (the local lord refuses the meeting but his steward secretly meets with the NPC and sends a message back to the PC that contains information helpful to the PC).

Now you will notice that there are a lot of various judgment calls in the above decision tree. Some of them are pretty tricky, but this is what makes narrating challenging and fun. The point of a decision tree isn't to tell you what decision to make but to help you make that decision.