Until about 20 minutes ago I didn’t know what interaction design is. Apparently the cool kids call it IxD. As it happens Interaction Design is what I have been specialising in for the past two years without even knowing what it is. I kind of like it when I arrive at a conclusion and then discover that some really bright people got there before me. It makes me feel clever.
So, obviously Interaction Design covers how people interact with things. In my case I am thinking solely about solo rpg rules. A player could by Mythic GME and use that one tool for years and be perfectly happy. There are tens of solo engines out there that are suitably generic to work with any game and you just need to pick the one that you like the feel of.
I tend to work from the other end. If you love a particular RPG and really get the way it’s mechanics work then I try and use those same mechanics to build a matching solo engine. The point of this is mentally you do not have to break off from the game’s mechanics to pick up the solo engine mechanics.
Take Blades in the Dark as an example. The game runs on pools of 1-3d6. Its mechanics are nice and simple. You roll your pool and take the highest die or dice. 4+ is a limited success, 5 is better, 6 is a full success and if you rolled more than one 6 then you have a critical success. No imagine you have to jump in and out of this system and Mythics d100 system. The two are wildly different and Mythic is much more table driven then Blades. (Shameless plug coming up here!) In my blades solo rules I have taken the standard Blades fortune roll as a core mechanic and used it as the oracle. You build you dice pool with 1d6 for unlikely events, 2d6 for 50/50 and 3d6 for likely. Where I cannot fit all the possible options in to the dice pool mechanic I still stick with the 3d6.
I feel I have been a bit unfair to Mythic here but originally Mythic GME did everything right. The GME used exactly the same mechanics as the Mythic RPG and the solo rules could be used for skill resolution. That is great interaction design. It is when you start to use Mythic for playing 7th Sea or even D&D that things start to split down the lines of two quite different rule sets.
So I think interaction design is really important but it is also entirely optional for solo players. Pick an engine you like and works for you or pick an engine that works for the game. They have the same end result if you are having fun.