In my experience playing Original Sin 2, it really depends on your party members and how invested they are in the story. Generally we've had a party leader who essentially leads what's gonna be happening that play session.

[No spoilers, just discussing game mechanics with a second player]

In early access, if you play with another player who is talking to a NPC, you can click on their party portrait and select "listen" so you can observe their conversation. During the conversation, when your partner has choices of dialogue, you can click on your preference of answer and your partner can see how you would like to reply. But ultimately, the conversation choices will be selected by the player who engaged in conversation first.

Ideally, you and your partner will be close enough that if the dialogue has a skill check, you can potentially help them out if you have something that could improve their roll. Also, if the dialogue ends in gaining XP, your partner wants to be close enough to get XP from the interactions too.