Still the question if the extra characters would add something to dialogues and cinematics too.
I don't see why this would be the case at all. Why would such things as dialogue and cinematics be dependent on party size? They are things that should be dependent only on each individual character.
Good question. We don't really have enough information available to us to answer it. It depends on 1) how they are triggering the banters and conversations and 2) how they have structured those conversations. In BG2 dialogues were based on a timer -- which gave the conversations a spontaneous feel "hey here we are in the street, it just occurred to me" but frustrated people who could never get to the end of the romances because they finished the game so quickly.
SoD used ground triggers -- which made sure that all dialogues fired and eliminated the need to keep the game running just to fire a dialogue but that system had the unintended consequence of lowering the replay value of the game. "Now when I step here, I have this conversation -- same story each time" ToB used a combination of event triggers and timers which was a better than just ground triggers but also made some dialogues predictable. In the infinity engine the number of characters didn't matter -- just which NPCs were present.
I dunno what system BG3 is going to use but if the trigger is linked to party formation or designed to start a specific four way conversation and five people are present that could, maybe, possibly cause problems. It all comes down to if / then statements and how they are designed. So this formation could cause problems. If astarian (player) [variable] showheart [variable] Wyll [variable] Lae'zel then [script var1 = var] + [script var2 = var] + [script var3 = var] + [script var = var ] = [script party disagreement] . NPC 5 wouldn't be included in the formula.