They already do this if you don't invite them to your group. The game gives you a chance to get to know everyone, if you don't want that, then just don't invite them. Shadow itself comes in grove. I am sure now Shadow is most developed character, she talks more, she has more questions and she goes wherever she wants. Other characters are still being worked on, but I'm sure they won't just stand in the same place.
As for the "special cases", well, conditionally, Gale and Astarion read books, Shadow prays, and Lae drinks. They have animations for this, but you don't get any benefit from it. And it's fine.
If you don't want them to just stand there, then just don't invite them, one day you'll find them somewhere else. You offer the same thing, but in a more difficult way, with a lot of extra work for developers and voice actors, because it will add additional dialogues which makes no sense...
