That's mostly what made BG2 companions so cool to deal with. They felt organic.
Bioware was also very clever in the way they built their story. Act 1 was tutorial. Act 2 was very open world and meant to gather your companions and define what kind of character you want to play. Act 3 and later were more linear with a stronger focus on the main story which meant less side quest and diversions. But that made it also easier for the developers to implement companion reactions (BG2 example: male character; relationship with aerie; when that female drow forced you to sleep with her). And the whole vampire thing at the end. And the relationship even continued in ToB. That's an entirely different level compared to many games where romance is just "One companion quest, sex scene, done".
But when Minsk entered berserk rage because of Aerie that was just another of these great moments I even remember more than 15 years later.