The first RPG I know of to use the phrase "play to find out" is Apocalypse World. It appears on pp 108-9 of the rulebook:
AGENDA
• Make Apocalypse World seem real.
• Make the players’ characters’ lives not boring.
• Play to find out what happens.
Everything you say, you should do it to accomplish these three, and no other. It’s not, for instance, your agenda to make the players lose, or to deny them what they want, or to punish them, or to control them, or to get them through your pre-planned storyline (DO NOT pre-plan a storyline, and I’m not [mucking] around). It’s not your job to put their characters in double-binds or dead ends, or to yank the rug out from under their feet. Go chasing after any of those, you’ll wind up with a boring game that makes Apocalypse World seem contrived, and you’ll be pre-deciding what happens by yourself, not playing to find out.
Play to find out: there’s a certain discipline you need in order to MC Apocalypse World. You have to commit yourself to the game’s fiction’s own internal logic and causality, driven by the
players’ characters. You have to open yourself to caring what happens, but when it comes time to say what happens, you have to set what you hope for aside.
The reward for MCing, for this kind of GMing, comes with the discipline. When you find something you genuinely care about — a question about what will happen that you genuinely want to
find out — letting the game’s fiction decide it is uniquely satisfying.
Page 109 continues with the following:
ALWAYS SAY
• What the principles demand (as follow).
• What the rules demand.
• What your prep demands.
• What honesty demands.
It therefore follows that there is no contradiction between
playing to find out and
prep (of a certain sort).
The rules for prep are set out on pp 136-47 (followed by an example). They include this, on p 143:
COUNTDOWN CLOCKS
A countdown clock is a reminder to you as MC that your threats have impulse, direction, plans, intentions, the will to sustain action and to respond coherently to others’.
When you create a threat, if you have a vision of its future, give it a countdown clock. You can also add countdown clocks to threats you’ve already created.
Around the clock, note some things that’ll happen:
• Before 9:00, that thing’s coming, but preventable. What are the clues? What are the triggers? What are the steps?
• Between 9:00 and 12:00, that thing is inevitable, but there’s still time to brace for impact. What signifies it?
• At 12:00, the threat gets its full, active expression. What is it?
As you play, advance the clocks, each at their own pace, by marking their segments.
Countdown clocks are both descriptive and prescriptive. Descriptive: when something you’ve listed happens, advance the clock to that point. Prescriptive: when you advance the clock otherwise, it causes the things you’ve listed. Furthermore, countdown clocks can be derailed: when something happens that changes circumstances so that the countdown no longer makes sense, just scribble it out.
For the most part, list things that are beyond the players’ characters’ control: NPCs’ decisions and actions, conditions in a population or a landscape, off-screen relations between rival compounds, the instability of a window into the world’s psychic maelstrom. When you list something within the players’ characters’ control, always list it with an “if,” implied or explicit: “if Bish goes out into the ruins,” not “Bish goes out into the ruins.” Prep circumstances, pressures, developing NPC actions, not (and again, I’m not [mucking] around here) NOT future scenes you intend to lead the PCs to.
There are two main ways that a
listed thing might happen. One is that, applying the principles in accordance with their agenda, the GM says something, and that something is on a clock. The other way is that, in accordance with the principles, the GM decides to
disclaim decision making (pp 110, 116) and thus advances a clock. (Always making a move that follows from the fiction: p 110-11.)
So there is at least one way - this way - of using
GM prep as an element in the process of
playing to find out.