From a comment in the blog post
Divinity – Original Sin revealed ! about the dialog system
so the idea is that in single player you can select a character type for the AI in which case the AI will answer in line with his character. Alternatively, you can opt to always have it your way, in which case the AI will not interfere. It's really up to you personal preference, though putting the AI on random will give you the most fun imho
I don't know if or how world interaction may be involved in quests, but previous Divinity games didn't have a predominantly combat focused quest design, and D:OS will have less combat overall than the previous games.
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