Sometimes I think it's whoever is closer to the person. Just experienced this with that tiefling by the telescope. Finished the fight with the bugbear and she started talking to Gale. So I reloaded and made sure I was closer. Moved everyone else.
It is whichever character is selected at the end of combat.
It still won't fix the problem. The fact that you can talk to any NPC in the game with every character in your party but the game will treat everyone exactly the same.
Unfortunately, no. Several of the dialogs are one-offs and cause approval/disapproval from other characters. If the MC isn't the one in the dialog, they do not get the option to cause the approval/disapproval. This bug has real effects on relationship mechanics.
It should be the question is is it possible within the limits of a video game rpg
I can see where it would screw up dialog trees if you had the option to switch during dialog and one character had already had the conversation while the other had not. The only controllable option would be a quick choice at the beginning of forced dialog to choose your spokesperson.
