They should use the passive skill checks, like the take 10 or take 20 rules from, what was it, 3rd edition. 10 is used for passive checks, take 20 assumes the character isn't pressed for time and this isn't a one time task. It's something that they could sit there and work on until they get it, like picking a lock. 10 or 20 plus your modifiers , then set their DCs where they want them for it to make sense. If failure isn't interesting, don't roll dice. This would work really well for almost all conversation options with companions, but wouldn't make sense for every check with NPCs. I would say there are some "checks" with NPCs that could probably become "choices" if the player has the appropriate skill. I think to take 10 in something, (or take 20 the player would need to be proficient with that particular skill.
On top of this, I feel if your whole party is present for a conversation, then all of their skills and abilities should come into play for that conversation. They should all be part of the conversation, all options anyone in your party would have should be presented and the skill checks should be done by the person with the best chance and the dialogue should be expressed by whoever has that highest chance for that option. That would really kick the dialogue experience up a notch.
