Most challenging but rewarding system you have learned?
Two games spring to mind: Hillfolk and various Powered by the Apocalypse games. Both were challenging for the same reason – they require the GM to give up some of the control normally associated with the position.
In Hillfolk I, as GM, have to take a turn in framing a scene just like all the other players at the table. In Powered by the Apocolypse game I have a role even more limited in many ways because I can only respond to what the players are doing, and can act decisively only when they fail a roll, which is unpredictable. In both cases the effect is roughly the same – I need to throw away the idea of a set-piece confrontation or difficulty, and come up with a lot more content on the fly.
In Hillfolk, as GM I sit and watch the action as it unfolds between the players, and get to interpret to some extent what success and failure mean, but I have no more say over the matter than any single player does. I am just “one of the gang”. Only when it is my turn can I really enact any sort of agenda of my own – but by the time things go around the table and back to me, the question that is most often on my mind is “where do I hit?” I have watched and listened to plots unfolding from all of the players, and of course each one has given me ideas about scenes of my own to enact. But there are multiple players and multiple scenes and unless I am very lucky and very creative, I can only throw a monkey wrench into so many of them – typically one or two. Which dangling plot thread should I yank on? Which provides the best story? Conversely, who isn’t tied in enough and needs a nudge to get pulled in? Can I tie two characters together in a plot line who aren’t already engaged with one another? Can I break apart two characters who are becoming too chummy? What needs pushing? What needs shoring up? I can’t do it all so I have to prioritize with a wide view of the story as it is unfolding, and a narrow view of which players need help to get involved and tied in. I only get my one turn just like all the other players – I need to make it count.
The Powered by the Apocalypse games are similar, in that I need to be thinking at all times about what will work best for the story, but I don’t even have a specific time set aside for taking my actions. I need to wait for players to fail a roll or ask me “what happens next?” for me to take a move, and the moves I can take are largely prescribed. I need to constantly be reassessing the plot and the characters – who hasn’t been engaged in the current scene? Who needs a nudge? What would be exciting and stimulating for the group? What is appropriate for where the group has moved the plot since my last move? And how do I execute the move? What narrative and mechanical resources do I employ? For Hillfolk the mechanic is so simple that I can use it any time, but do I want to set up a one-hour fight in Dungeon World when I make a move, or do I want to fiat it and say “this happens” and deny player agency in affecting the outcome for the sake of the plot? Are we near the end of the session so I should be setting up a cliffhanger, or are we in the middle of a session where I need to provide something that will move the plot along?
Questions, questions, questions. And questions are a good thing. Having to act on the fly keeps the narrative fresh, keeps me as the GM interested and focused, and allows everyone in the group to take advantage of unexpected results to move things in a desired direction. When you sit down to run a D&D module you have a pretty good idea of where the plot is going to go – you have the dungeon, you have the monsters, and overall the player characters are either going to win (probable) or lose (improbable). For Hillfolk and the Powered by the Apocalypse games you are the William Paley’s god – you create the watch and then throw it out the window to the players. From the moment play begins it is they who have control of what happens to the watch – they can let it tick along as you intended, they can monkey with the workings and turn it into a mechanical music box, or they can smash the damned thing on a rock, build a quantum powered star-ship, and fly to Alpha Centauri. And all of those things are okay. Your job as the GM isn’t to set up the plot and then let them run through it, it is to set up some parameters so that everyone understands where they are starting from, and then let those parameters stand, fall, or be modified according to the desires of your play group.