There's a zoom dialogue option in the options menu. You can try playing with it to be sure, but I suspect it's mainly to enable/disable zooming on conversations for the player not doing the talking in co-op games.
Can someone confirm this for me? I dont have the game, just saw it at a friends house and youtube.
Ok, I just tried this option out for you and it's the complete opposite of what you want. It's like a super-zoom on conversations.. much closer than the normal zoom. When you activate it, it pops up a warning that it's intended for mods, not the main campaign.
Sorry.
