First, by analogy: a film-maker, novelist or playwright
addresses their theme. The reader of their work (subject to general caveats about the author being dead etc)
explores the theme. For instance, when I read LotR I am presented, by JRRT, with his ideas about the relationship between free will, providence, and resistance to evil. I don't, myself, in the act of reading,
address the theme.
Second, directly: when the GM presents a situation with an intended or envisaged "correct" answer - probably the most classic example in D&D play is GM-adjudicated alignment, in which the GM has already decided what counts as
good or
evil - the players are exploring (in the stipulated sense) that theme. Vincent Baker sets out the contrast with
addressing theme in this passage from DitV (pp 143-4):
In most RPGs with religious content, the GM arbitrates the characters’ morality. The GM plays God (or the gods) as an NPC, giving and withholding moral standing — whatever form it takes in the particular game: Faith Points, Alignment Bonuses, whatever — based on the characters’ actions. Not in Dogs.
In Dogs, the GM has no opportunity to pass effective judgment on a PC’s actions. Talk about ’em, sure, but never come down on them as righteous or sinful in a way that’s binding in the game world. The GM can’t give or withhold dice for the state of a PC’s soul, and thus never needs to judge it.
Which is good! Which is, in fact, essential. If you, the GM, can judge my character’s actions, then I won’t tell you what I think. I’ll play to whatever morality you impose on me via your rulings. Instead of posing your players an interesting ethical question and then hearing their answers, you’d be posing the question and then answering it yourself.
In the same vein, this is why:
There cannot be any "the story" during Narrativist play, because to have such a thing (fixed plot or pre-agreed [resolution of] theme) is to remove the whole point: the creative moments of addressing the issue(s). . . .
It all comes down to this: a "player" in a Narrativist role-playing context necessarily makes the thematic choices for a given player-character.
As well as formal cues like alignment rules, or - again a classic in D&D - the GM playing the gods and telling the cleric player what counts as adhering to or departing from divine requirements, there can also be myriad informal cues or expectations that push play towards exploration of, rather than addressing, theme and its resolution. I would conjecture, based on what I read online, that the two most common of these are (i) expectations that the players will pick up on the GM's hooks and follow along with the GM's material, and (ii) expectations that the players will play an essentially cohesive and cooperative party of characters.