The player is acting as more along the lines of the "gut" feelings of the character rather than direct control of what they're saying or thinking.
There are multiple factors involved in what is being agreed to and how it would work. For example if the the player selects "agree" it doesn't necessarily mean that the character 100% agrees, but rather that the gut feeling with regards to what the other character is saying is "true"... However, the character could act deceptively (even without you telling them to) or agree emphatically, or just listen till there is a disagreement to jump on. This all depends on the context of what is being said and who is saying it. This is because if you're in the presence of an idol who you disagree with you might just agree with them because you love them and want to date them and likewise you may also be in the dark lord's presence and try to fool them into thinking you're on their side or you may be in a meeting with the rebels and you're considered a leader so you need to more emphatically support what was said.
The best way to describe what I'm talking about is probably something like Context Sensitive Dialog Mechanics.
Character opinions are never spelled out directly to the player, however, they are tracked by the game and are changed through various means.
Think of it sort like Fable's sliding scale of good vs evil, but with multiple vectors and every character/npc has various settings about how they feel about these topics which influences what they say to the player character which influences how they respond which influences what the NPCs thinks/says and so on and so on. The goal is to create a dynamic dialog system where player choices change the information they are given, but characters (including player characters) act more naturally and more in line with those specific characters. So that the main character acts like the main character even when the player is moving them down a given direction and the change in the character actions/reactions is gradual rather than suddenly this way or that.