As all the followers appear before you after you defeat the final boss, almost all of their conversations are available until the end of the game, but there are a few that can be missed so I'll mark those with a +. Many of the conversations will unlock only after having heard other previous conversations, but will not become available until the point specified. Also, some of the follower conversations may be hidden if they are not following you, but by having them follow you, you can get a complete list (without the extras).
Act I
- Becoming a Templar: Available after accepting the quest The Fallen Star by starting Act I until the end of the game.
- Sins: Available after accepting the quest The Broken Blade until the end of the game.
- Initiation: Available after accepting the quest Trailing the Coven until the end of the game.
- Lessons: Available after accepting the quest The Imprisoned Angel until the end of the game.
Act II
- Seeking the Truth: Available from when you are given the objective Go through the Sundered Canyon during the quest Shadows in the Desert until the end of the game.
- A Dangerous Journal: Available after accepting the quest The Road to Alcarnus until the end of the game.
- Confronting Fears: Available after accepting the quest A Royal Audience until the end of the game.
- The Past Revealed: Available after accepting the quest The Black Soulstone until the end of the game.
Act III
- The Torn Page: Available after accepting the quest The Siege of Bastion's Keep until the end of the game.
- The Templar's Research: Available after accepting the quest Turning the Tide until the end of the game.
- + The Truth: Available after accepting the quest The Breached Keep until you complete the quest The Breached Keep.
- Secrets of the Tome: Available after accepting the quest Machines of War until the end of the game.
Act IV
- A Templar's Lesson's: Available after accepting the quest Fall of the High Heavens until the end of the game.
- + Adria's Betrayal: Available after accepting the quest Fall of the High Heavens until you accept the quest The Light of Hope.
- Shadows of the Coven: Available after accepting the quest Fall of the High Heavens until the end of the game.
Extras
These don't count toward the achievement
- A Hero's Honor: Available from when you are given the objective Talk to Auriel and bring your quest to an end during the quest Prime Evil until the end of the game.
- Diablo Vanquished: Available from when you are given the objective Talk to Auriel and bring your quest to an end during the quest Prime Evil until the end of the game.
- Fallen Comrades: Available from when you are given the objective Talk to Auriel and bring your quest to an end during the quest Prime Evil until the end of the game.