I would inspire myself to existing chatbots
These are just two examples that came to mind, I hope they help:
- Emacs psychotherapist a rather old one, asking questions on the previous statement with the purpose of making the user think about their issue
- A.L.I.C.E. a more recent one, not particularly intelligent in any way, yet more natural than the one above
It seems to me that the common feature is that they lack emotional response. They can be very articulated, evolve a conversation taking into account a long history of sentences, but, in my opinion, they lack empathy.
"I am sorry to hear that" answers anything sad.
"I am happy to hear that" answers anything happy.
A human being could just cringe, or change topic, or modulate their emotional response throughout the conversation. It is entirely possible that a human character would try to place some distance after hearing five consecutive disgraces that occurred to the other person. A chatbot may remain in their static emotion, repeating the same type of response, or cycling through a predefined "optimal" set of answers. It does not have to feel clumsy, just eerily detached.