It is possible to disable dual dialogues in the Game section of the options; check that 'Dual dialogues' is enabled. If disabled, only your lead character will respond when speaking to NPCs and you will not get the ones between the main characters.
During character creation, did you set an AI personality for either main character? That would have the non-lead character answer automatically in dual dialogues, though they should still say if they agree.
Make sure you read the diary that the guy that jumped off the clif has on him; just discovering the body doesn't trigger the dialogue.
