Can you contribute to conversations in co-op mode? You can suggest only, vote. Same as in EA. Reason: System was not tenable Whoever initiates the convo the rolls will be on that character? Yes - same as EA
It does seem weird. The argument Swen said was that they had another system implemented but it was unattainable. But, that isn't what the question specifically looking to answer.
I think the current system is fine when you can make decisions when you can choose who to speak with, but situations where dialog actions are forced by some odd interaction, like combat end, that we want to be able to choose who can speak.