There is never actual conversation between Shep and the two DLC characters (Zaeed + 1 other). They just get a room on the ship and will talk at you like that.
Can't speculate why obvs but I'm sure it would have been a ton of work to make them fully interactable at all points in the story
Surprised by the negativity in these comments (Well, no, that's a silly sentence)
This thing certainly hasn't aged well visually (neither technically nor stylistically), but there's a heck of a game under there. Strongly recommend not reading this book by its cover.
Jeverage's comments