That’s exactly it, they have the ability to go about certain scenes in different ways.

I still only have the Astarion/Cazador example, because that’s the only origin character I played as, but one moment I remember as different between Astarion as party member versus Astarion as player is the conversation with his former lover Sebastian before confronting Cazador.

In my playthrough with Astarion as an NPC, he basically auto-piloted through most of that conversation. He remembered who Sebastian was, it was somewhat touching, and the player was able to ask Astarion questions for context.

In my playthrough as Astarion as the player, you can navigate that conversation in different ways, but there are also certain things that are no longer a given. You actually have to roll to remember who Sebastian is (which I flubbed) and that changes the course of the conversation away from the NPC “default” conversation.