Most arguments that I could contribute have already been made. Personally, I'm conflicted on the issue, because on the one hand, I firmly believe in the improv comedy "yes, and..." rule, but on the other hand, there's a lot of baggage, much of it already being explored in this thread, to navigate.
One thing no one has mentioned is a gentler version of the "you must be crazy" method. You don't necessarily have to go so far as to call someone out as a raving lunatic. It's possible to just have your character openly disbelieve whoever's playing the NPC, thinking they're either a good impressionist or just stubbornly refusing to believe that they'd ever experience the equivalent of running into Johnny Depp at a coffee shop. If that's as far as you push it, you get a sort of Schrodinger's NPC situation. They could be the NPC, they could not. It doesn't matter either way, because you don't believe them. And on the other side, it doesn't matter whether you belief or not, because beliefs can contradict factual reality. This way, you dodge the uncomfortable "becoming besties with Minfilia" situation without directly contradicting their rp. Both players' "realities" can coexist long enough to get through one or two rp sessions, and after that, you just don't need to involve yourself in longterm rp with that player.
Of course, if players begin doing things like using their power to suddenly siccing an entire squad of soldiers on you or branding you a criminal, that starts approaching "sir, you're doing things to my character that I don't approve of, please stop" territory, and likely where I would start drawing a hard line, especially if it's walk-up rp with someone I'm not familiar with. Those are consequences that reach far farther than that one-off rp ever will, and the non-npc player is likely going to have to play them out on their own.
That said, if I ever saw a bleeding Primal walk into the Quicksand, I might take umbrage, as that goes so completely against established Primal behavior (not to mention raise serious questions abut the efficacy of Ul'dah's guards) that it becomes lore breaking.
One thing no one has mentioned is a gentler version of the "you must be crazy" method. You don't necessarily have to go so far as to call someone out as a raving lunatic. It's possible to just have your character openly disbelieve whoever's playing the NPC, thinking they're either a good impressionist or just stubbornly refusing to believe that they'd ever experience the equivalent of running into Johnny Depp at a coffee shop. If that's as far as you push it, you get a sort of Schrodinger's NPC situation. They could be the NPC, they could not. It doesn't matter either way, because you don't believe them. And on the other side, it doesn't matter whether you belief or not, because beliefs can contradict factual reality. This way, you dodge the uncomfortable "becoming besties with Minfilia" situation without directly contradicting their rp. Both players' "realities" can coexist long enough to get through one or two rp sessions, and after that, you just don't need to involve yourself in longterm rp with that player.
Of course, if players begin doing things like using their power to suddenly siccing an entire squad of soldiers on you or branding you a criminal, that starts approaching "sir, you're doing things to my character that I don't approve of, please stop" territory, and likely where I would start drawing a hard line, especially if it's walk-up rp with someone I'm not familiar with. Those are consequences that reach far farther than that one-off rp ever will, and the non-npc player is likely going to have to play them out on their own.
That said, if I ever saw a bleeding Primal walk into the Quicksand, I might take umbrage, as that goes so completely against established Primal behavior (not to mention raise serious questions abut the efficacy of Ul'dah's guards) that it becomes lore breaking.
All credit for the graphic to Benedict Whiteraven, you magnificent bastard.