I don't see why you need a dialogue window to talk to the guy.. What's wrong with just RPing with text appearing above his head?
Also, I'll thank you not to make erroneous presumptions about me.
Its RP, not PvP, not PvE, not chatting. The best RPers should actually become GMs who play spawn characters for events lead through RP. Interacting with players who have style ( not necessarily rich/great PvPer) and can handle creative thinking more than riddles and mini-tournaments. A series of events for instance, almost like quests except more custom on-the-fly. Eventually you will come up with a few good events you can have set in as real quests maybe. Without some free-RP you are left with forcing the admins to come up with riddles etc etc.
I think even admins would enjoy taking a bit off their work load (its also a game to them too remember). This allows players to do more between them without relying on these events so regularly since they can spontaneously stir up shit between them through RP. Player to Player lead RP that could offer ideas to the admin or keep players at least occupied so I don't have to hear people complaining about admins not coming up with anottther riddle or event. Worst thing you can do is make this game feel like a job to the admin.
Better off allowing the players to take more into their own hands, stealing while bartering shouldn't be possible since they are messing with their inventory as its going on. (stealing while just Chatting is fine even in full dialogue imo) If you get into combat... automatically the fullscreen dialogue should lower. Not that it matters players die so fast even if you are on fullscreen, have 12 words over your head or you're not even looking at the screen? You're dead before you know it for the most part.
If you hear bullets, exit and get ready for action? I don't see the big issue. I doubt anyone who doesn't RP would bother but I could see it catching on if people actually go out to RP and then they could easily come up with new quests/events we could hardwire into Fonline eventually if players can write up some good stuff.
if I can figure out how to post a picture (button but no upload?) I'll show you, since its much easier than all this explaining. (Fake possibility edited with paintshop:)
Not to mention it doesn't seem like much work to implement being there is already a say button/command so should be able to easily mimic it into an emote button/command.