Optional conversations are short side chats you can start with companions or trigger at objects when a prompt icon appears. They never change the plot, but starting every one earns the Getting to Know You trophy in Part I.
The Quarantine Zone has chats with strangers near the Area 5 gate and with Tess outside the wall. Bill's Town includes Ellie at an arcade cabinet and the chess board. Pittsburgh is the busiest chapter: the hunters' garage, ration graffiti, a fashion poster, the hotel coffee machine and a Hawaii photo backdrop all trigger talks. In The Suburbs you can chat with Henry about barricaded houses, ask about an ice cream truck, and throw darts with Ellie and Sam. Bus Depot has several more in its Highway Exit section.
Many prompts are easy to walk past, especially around the Hotel Grand. The trophy needs every one, so talk to Ellie whenever the icon shows.