I walk past the building where T's office is, on my way to and from work every day. I have met him a few times when I've been on my way home. Once I was taking the bus to another part of town for a meeting after work, and was standing by the bus stop conducting a conversation with T inside my head, when he came walking past. That startled me.

I would never strike up a conversation or say anything other than "Hi" to him outside his office. But I do greet him, and he greets me, same as you would any acquaintance you run into in the street.