I think it's made pretty clear that the party interacts and picks up on what you talk with others about.
Shadowhearts "I couldn't help listening, well I could" is a good example.
Oh right, I totally forgot about that! Now that I think about it, there are also other examples of them listening in on each other, and all in all it's not strange that they would (though it'd make at least half of them giant hypocrites, since they seem to value privacy so much, but then again most of them aren't exactly moral people anyway). Since it's addressed, I guess it's okay to keep imo, it just surprised me that ex. someone as guarded as Shadowheart would open up to you if you persuade her and then not be careful to keep it from the others. But I guess PC = whole group when it comes to what people reveal to you and not. Everyone seem to know about Gale's condition after he tells you in private as well.
I haven't made it very far into BG3 yet, but how do the companions normally treat and discuss things amongst themselves? Would they discuss current events or talk about things that occurred in their day with each other? Approval and disproval can certainly be an annoying mechanic, but it sounds like it's done more realistically here than it was in DAO.
It's never actually discussed on-screen between companions, but due to them approving/disapproving whether they were present or not, it's kind of assumed that they do.
