Will companions be interchangeable during long rest? Yes, at the start of your adventure your recruited companions will be at camp when not in the adventuring party, and can be swapped in and out at camp. Just like friends in real life! After the first act however you are going to have to commit, also just like in real life.
I love this game until today, but losing the ability to change your party after act 1 means I will not spend any money on it! Because it means you only have effectively 3 companions which is a joke compared to every single party-based CRPG I have ever played: you guys said you listen to feedback so I really doubt anyone thinks this is good so please post here to make sure Larian changes this ASAP!
Losing party after act 1 was hated by everyone in DOS2 and no other RPG ever did it because it means the player loses the choice to build the right party for each encounter, meaning the game losing depth!
