Well, if you listen to some of the datamined voice lines, some of them are the characters narrating their own feelings/responses or how they see a situation. I've played through the game twice (well, two and a half times, the half-playthrough encountered bugs with Gale's romance which I didn't finish). So while I know the situations, I am looking forward to the characters' thoughts on them. Yes the characters are forthright in their opinions, but not ALWAYS, there is always something more under the surface of what they say vs how they feel, and it's this difference in perspectives that I look forward to.
That said, it's probably best to play as your own character first, and as the origin characters second, so this aspect does become enjoyable rather than stifling one's gameplay.
For example, if you watch Astarion's illithid dream scene - which is incomplete/unreleased/glitchy, but someone has datamined it on Youtube -
he dreams of Cazador and his commandments, gets told by the vision that he is worthless, then when he shakes the dream off, he wants to see HOW much control Cazador still has over him, and is curious what would happen if he bites one of the other companions. He does this not out of hunger but in defiance of Cazador, to see if he even can break the rules, to test whether the leash is still there so to speak.
In contrast, in my playthrough as my own character, I got the bite scene immediately after his Stargazing scene, which I thought fit together very well. BUT. My perspective on the bite, and my understanding of his spoken explanation "I feel weak, but with your blood I will be able to fight better" vs Astarion's perspective "let's test this theory, is he still in control of me? Can I really drink the blood of thinking creatures - and what happens if I do?"
is exactly the change in perspective that I enjoy finding out more about. This is what I hope there will be more of when we are able to play as the characters.
I don't see it as limiting my play style (I have my own player character for that!), personally I see it as an opportunity to explore the existing characters deeper, and to step out of my comfort zone when playing. I stepped out of my comfort zone already doing the Evil playthrough (right after doing a Neutral-Good Tiefling Druid playthrough, aaaaagh) - and while it made me feel awful to be slaughtering the NPCs my other character had worked so hard to save, it was still interesting to experience the different view and different feeling I got from this path. It wasn't a GOOD feeling, but it was different, and I don't want to play the same game the same way all the time.
Also, the characters' reactions DO change with you already whether you choose the Good or Evil path.