Leah only appears to discuss quests and the same three topics before the big incident, after which she only speaks of her power. When can we dialogue about "Tristram", "Leah's Childhood", and "About the Temple"?
5 Answers
"Tristram" is available during The Fallen Star
"Leah's Childhood" and "About the Temple" first show during The Broken Blade, but are also available at the beginning of Trailing the Coven
To get "Tristram" do the level in the first act where you have to go to Adrias hut, and into her underground lair, kill the ghouls etc and talk with leah to complete the quest, after the quest is completed, talk with her again and the Tristram dialogue should be there.
I was able to get all of these conversations by the second time she is in your party, in Act I (when heading to the Drowned Temple). Specifically I remember very clearly 'About the Temple' didn't show up until I was inside.
I got "About the temple" as an option only in the drowned temple itself.
Jump to the Quest "Broken Blade" and "Talk to Alaric", you then have to go to the festering woods to retrieve the two beacons. Then you can enter the temple and after Leah says something about "sin" you can talk to her about the temple (right-click her picture). This is the way it worked for me.