Either make only the main character be able to lead dialogues
And what happens if they're dead, but the NPC you just saved wants to initiate a conversation ending the quest?
at least make dialogues dynamic which means that NPCs should remember that they already talked to your party (no matter to which char) and refuse to talk to your party again about the very same matter.
Then what about party members that were absent during the previous attempt?
And in both cases, all mechanics must remain consistent between SP and MP, otherwise they'd be effectively making two games(identical, but separate).
Once this system is integrated your companions will be AI-controlled in such decision situations (like it was in DOS1 for your second character).
Which was optional, and as such it should remain.
