You can change the AI once you unlock the living quarters in the End of Time.
Until then, if you start dual dialogues with the 'random AI' one as leader, you control their dialogue options. Alternately, you can leave either main character too far away to participate in a conversation, if you don't want the random selections to interfere when talking to an NPC (such as when trying to recruit a companion). Finally, you can disable dual dialogues in the options.