You brought up DOS2 as an example, but that's not really true that all companions could be 100% influenced by the player there.
They had very strong opinions and urges at first, you barely could convince them in anything. For example, there was no way to stop Sebille from killing certain character at the start of the game.
Later with grows of hidden approval meter you could influence them, but they still would have some principles. For example, you could get pretty evil Ifan, but don't you dare to use the deathfog machine on Arx with him in the party. He's soo strongly against using deathfog that even if you're lovers he would go hostile.
I'm expecting something similar with companions in BG3.
IIRC those "principles" were pretty barebones. I think it was just the deathfog with Ifan (and Beast? Maybe Sebille?). To me, with one action that might make your team stop, you could turn altruistic heroes into self-serving sadists pretty easily.
