Trust me, talking protagonists is not a good idea. You'll end up putting words into player's mouths that they themselves might not agree with. And quite possibly, that could ruin the immersion for some people. Like in that one game where my character showed a lot of trust and respect toward a character that I completely hated and wanted to kill with fire. That killed the entire game for me. Never bothered completing it.
If you're really going to go down that route, though, I'd recommend giving the player some choice in the conversation. I know that you said that you didn't want to write out a whole bunch of dialogue trees, but you should at least consider having small instances where the player is able to choose a response more fitting. Like, if someone is talking about science, a player who's into that kind of stuff could say "Hey, that's cool!" and the guy talking would reply "I know. Isn't it incredible? whereas a player who really couldn't care less about science could go with "Yawn. Boring." and the guy talking would reply "Yeah, well, so are you." or something similar. You wouldn't have to deviate the plot at all. It would simply add a little more depth and interaction into the world, rather than it just being a static region where everything happens the same every time you play it.
Now, I'm not trying to tell you guys how to make your game - I'm just suggesting something that might make it a bit more interesting.