GM says, "Roll!" Player says, "I got a 20! I get to narrate!" (launches into long and involved monologue about how this opponent is really his long-lost mother, to the consternation of the GM who'd been playing the NPC all along as someone totally different, say, Barnabas the stablehand) The GM is now forced to junk 80% of his prep and re-write the whole scenario in the next microsecond as the player looks at him expectantly.
I'm saying, this isn't what most people are talking about, when we talk about non-railroady Narrativist play. This is kind of a consensual-storytelling, make-it-up-as-we-go, round-robin type thing. Frankly, it's pretty boring in most circumstances and tends to create wandering, meaningless pseudo-narratives.