Well, I am looking from a single player perspective if an OC has a single decision tree, it's totally based upon the actions that single player took in their game.
In a multiplayer game, the same single decision tree reacts to the exact same situation. This single decision tree would never fire unless my OC character is at that position at that time. If I am not, that OC-specific tree is just lost forever, thus making that decision of never doing something in that tree forever a flag in the game state, thus can trigger other actions.
But this is completely independent of if we are playing a single player game or multiplayer game. The only thing that I could see different is if the OC is a companion, the OC will react/make decisions based upon what the actual PC player will do. IN multiplayer, that same OC that is controlled by a player will have the same decisions to make as if playing a single player game, playing that OC.
This is why decisions of OCs are predetermined based upon actions by the player, but if actually played by a player, those decisions are no longer predetermined and thus the same events would give a dialog choice to do. But, basically, the dialog choices are only for any OC-specific events, thus will be not too much different than a Tav PC.
An example, let's say Wyll, has an event in camp that Mizora comes out and tells the party that she wants them to kill an NPC.
During a single player game with the player playing anything other than Wyll, this plays out as you expect, with Mizora and Wyll dialog along with the party talking, you making decisions.
During a single player game with the player playing as Wyll, this plays out exactly the same way, except it's just Wyll making decisions on what to do instead of Tav reacting/talking and seeing reactions from Wyll, but the outcome is the same based on the same dialog decisions.
During a multiplayer game with Wyll as a companion, would play out no different than a single player with Wyll as a companion.
During a multiplayer game with Wyll as the PC, it would play out exactly the same as a single player game but as Wyll being controlled by the PC.
You can see, in these situations, the there an illusion that there would be odd decisions/interactions but, really, the exact same set of trees and decisions are made at the exact same time, the only thing that changes is what dialog would happen. But this is probably why Larian makes the PC dialog have no voice over, or else the VOs would be too large. But, mechanically, this isn't too bad to perform.
Of course, this is all speculation.
