Is there a way to change which character is leading a conversation? When playing in co-op one of us will sometimes start a conversation before realizing a different character's abilities would work better. For example, if a Charisma check comes up I'd like to switch to the character with higher Charisma, as I would be able to do in an actual game of D&D.
You can't, once the conversation has already started.
However, it is a highly requested feature (see for example here), so there is a decent possibility of it being added in the future, or a mod being created.
[Sept 2023] There's now a mod that makes it so the highest modifier in your party is always used. That's not quite the same as switching characters, but it's better than nothing.
You can do it, but only in advance. The character in the current party which you have currently selected and therefore goes in the front is the one who will have the conversation. So for example if you suspect that a conversation with an NPC will entail Deception/Persuasion checks, you can let Astarion take the lead.
The cleric cantrip Guidance as well as the wizard cantrip Friends are also very useful to use. During the die roll part, there's an "Add bonus" button at the very bottom, where your party members can cast spells to improve your chances.
It's possible to interrupt a conversation, which will then allow you to restart it with a character of your choosing.
When Shadowheart was picked over my druid (bears don't make great conversationalists, apparently), I was able to swap to my druid, then use
Shove to yeet her off a cliff. Naturally, this interrupted the conversation flow, and when my druid then spoke to the recently de-petrified wizard, he seemed none the wiser.
Your mileage may vary with this tactic, as it is only possible when not all of the characters are participating in a conversation, and your ability to disrupt the conversation might likewise vary. (I'd presume a gnome would have a much harder go at pushing Shadowheart, off cliffs or otherwise!)
Yes, you can change which character is leading a conversation in the game. Open the conversation interface and select the character you want t show the discussion. The interface usually appears automatically when you initiate a conversation with an NPC, but you can also bring it up manually by pressing the corresponding key or button. Once you have selected the character you want to lead the conversation, their abilities and stats will be used for any checks or decisions that come up during the dialogue This can be especially useful if you have multiple characters with different abilities that you want to utilize during conversations.