In multiple versions of the story, the therapist character will ask Stefan if he wants to talk about his mom. You should do this because if you don’t, the story will just try to make you do it anyway. Also, when you do find out about Stefan’s mom, and the missing toy rabbit incident from his childhood, things start to get a little more interesting. So, talk about mom in all versions.
However, if you get to a point where the dad takes Stefan to the therapist and Stefan sees Colin on the street, always, always choose the option to follow Colin. Do drugs with Colin. This will give Stefan the ability to “time travel” through a mirror at a very important point.
This point occurs when you find Stefan’s dad’s secret room. When you find that room, there’s a safe. If you use the word “TOY” as the password, you’ll get the most bittersweet ending available. Stefan is able to go back in time and as a five-year-old, join his mother on the train that is doomed to be derailed. This effectively kills Stefan in the present. It’s sad, but it’s the only version that doesn’t lead to patricide. However, prior to this point, when Colin asks who should jump off the building, you obviously have to pick Colin, because if Stefan dies, the story just ends.