What I think is that it should be where the Player Controlled Character is acting as the initial face BUT you can often choose another party member to do a check.
For example, say we have to persuade someone to let us pass without shooting us, if Astarion is nearby we should be allowed to let him make the check and convince the guy to not attack. This coupled with the aforementioned ability for companions to have a say in conversation would make the party feel more like a party where everyone is cooperating and contributing.
