I think
@TwoSix was alluding to something like
this:
I say, "my character, this guy in Thatcher's london, who has everything to lose, he goes to his lover's flat and convinces him to keep their affair private." You say, "y'know, I don't think that his lover is inclined to keep their affair private, do you?" And I say, "no, I suppose not, but my character is desperate to convince him anyway. In fact, he brings an antique revolver with him in his jacket pocket, in case he can't."
(Look, just look: the character has no "character sheet," but he's a whole character, fully realized. I can play him effortlessly.)
How do we decide what comes true?
We can simply agree. That works great, as long as we really do just simply agree.
We could flip a coin for it. Let's do that: heads my character convinces yours to keep their secret, tails he murders him instead.
Or y'know, that's a lot to deal with. Let's have a rule: whenever a character's life is at stake, that character's player gets to call for one re-flip of the coin.
On the other hand, isn't my character's life at stake too? His wife, his kids, his position, his money, his everything? Which should have more weight between us, your character's life or my character's "life"? Shall we go best two of three, or is that setting life and "life" too equal?
How about this: we'll roll a die. If it comes up 1 or 2, your character will refuse and mine will kill him; if it comes up 3-6, your character will agree to keep the secret and (unknowingly) thereby save his life. It's unequal because my character killing yours is less to your liking than your character ruining mine's life is to mine. It's unequal to be fair to us, the players.
Notice that we haven't considered which is more likely at all. We probably agree that it's more likely, in fact, that your character will refuse, so my character will shoot him. But that doesn't matter - either could happen, so we roll according to what's at stake.
Also, notice that we aren't rolling to see whether your character values his life in the face of my character's gun in any way. We're rolling to see if your character agrees to keep the secret without ever knowing about the gun, or if he refuses without knowing about the gun and my character shoots and kills him.
In other words, (i) the participants clarify what is at stake in the situation, and then (ii) identify if they disagree on how it should unfold ("what comes true"), and if they do (iii) toss the coin, roll the die or whatever.